By: Mike Caviston, Director of Fitness, Naval Special Warfare Center
Posted: March 4, 2020


Short Interval runs

The goal of the Short Interval (SI) format in the Naval Special Warfare (NSW) Physical Training Guide (PTG) is to build up to performing 8 x 400m (1/4 mile) at the fastest pace that can be sustained for the entire workout. That is very demanding both physically and mentally, but a very effective means of improving your performance on the 1.5-mile run portion of the Physical Screening Test (PST). However, many SEAL and SWCC candidates make fundamental mistakes when executing the SI format regarding the number of intervals they start with; how fast they try to go when just beginning training; how consistently they pace the workout; how well they recover between intervals; and how quickly they try to improve from week to week.

Example: Typical formats include running 400m (1/4 mile) repeats, allowing a recovery period of 2-2.5 times the amount of time it takes to perform the work interval. Initially, your intensity or pace should be slightly faster than the pace of your most recent 1.5-mile run. Your 400m interval pace should be about 4 seconds faster than your base pace.

Long Interval runs

The Long Interval (LI) sessions typically involve up to 30 minutes of total work (not including recovery) in 1-4 intervals. These intervals typically involve moving for approximately 7-20 minutes without stopping at a pace approximately 90-95% of the maximal pace you could hold for that duration.

Example: If you can run 1.5 miles in 9:00 (6:00/mile pace), your LI running workouts would be around 6:20-6:40/mile pace. The workout should be very demanding but not totally exhausting. On a scale of 1-10, with 10 being the greatest effort possible, the workout should feel like 8-9. If you're not sure what your pace should be, don’t overthink it. Work hard and try to get faster over time.

Long Slow Distance runs

The intensity of Long Slow Distance (LSD) work is low to moderate, so your pace should feel somewhat relaxed. These workouts build endurance and provide relative recovery between more intense sessions. To determine the appropriate intensity, use the Talk Test. You should be able to talk comfortably in short sentences while training, drawing breath between phrases. If you can’t speak, you're working too hard, and if you can speak continually, you're not working hard enough.

Here's what to do: Focus more on duration than intensity. Forty minutes of continual running is typical. A beginner may need to start at twenty minutes, and someone who is very fit might perform 90 minutes of continuous movement in one session. A practical goal is to build up to comfortably running 8-10 miles without stopping.


SI - Total miles: 8

  1. Warm-up 3 miles
  2. 4 x 1/2 mile for 2 miles
  3. Active recovery between intervals 1.5 miles
  4. Cool-down 1.5 miles

LI - Total miles: 10

  1. Warm-up 3 miles
  2. 3 x 1.5 miles for 4.5 miles
  3. Active recovery 1 mile
  4. Cool-down 1.5 mile

LSD 1st session - Total miles: 11.5

  1. Warm-up 1.5 miles
  2. Run 10 miles

LSD 2nd session - Total miles: 9

  1. Warm-up 1.5 miles
  2. Run 7.5 miles

Total miles: 38.5 miles

This total is close to the 40 miles per week that some have suggested is necessary to prepare for BUD/S or BCS. The example above is for an experienced runner who has been training for 6 months or more.

The PTG provides sample schedules that build running mileage up to about 32 miles per week over 26 weeks. Beyond 26 weeks, more frequent running and more total mileage may be tolerated.

While running fitness is critical for success, it's important to build running mileage gradually and progressively to avoid overtraining and injury. To develop greater cardiovascular fitness while avoiding overtraining, supplement run and swim training with cross-training activities like cycling or rowing.

Components of an effective running program:

  1. Weekly high-intensity interval workouts
  2. Weekly threshold workouts
  3. Two or more long, steady-state workouts per week


These sessions alternate short, intense work intervals with periods of recovery. The format begins with running 4 x 400m (1/4-mile) intervals, allowing a recovery period of 2-2.5 times the amount of time it takes to perform the work interval, and progressing to 8 x 400m (or the equivalent). For your first Short Interval workout, your intensity or pace should be slightly faster than the pace of your most recent 1.5-mile run.

Short Interval pace example:

For running, your 400m (1/4 mile) interval pace should initially be about 4 seconds faster than your base pace for 1.5 miles. For example, if you recently completed a 1.5- mile run in 10:30 (a 1/4 mile base pace of 1:45) then your interval training pace should be about 1:41. If your 1.5-mile time is 10:00, your pace for your first INT session should be about 1:36.

Regardless of initial fitness, begin with 4 intervals for your first session. Build progressively toward completing 8 intervals. Don't run more than 8 intervals during an SI session. When you can complete 8 intervals in the prescribed times, work on gradually performing the intervals a little faster each week.

Work on consistency, trying to keep little variation between your fastest and slowest interval and pacing yourself to be fastest at the end of the workout. On different training days, for variety, supplement 400m intervals with other distances from 200m to 800m (see the PTG for examples).

The total distance of all work intervals should be no more than 3200m (2 miles) for the SI format. Allow yourself enough recovery time between intervals to maintain the proper work intensity, without taking excessive time or wasting time. To promote faster, more complete recovery, use some active recovery, such as brisk walking or easy jogging for part of the time between intervals.


In an older format of the PTG, the LI workouts (called CHI, for Continuous High Intensity) were prescribed by time. This was done because I thought it would be easier for most people to keep track of time when it might not be possible to accurately measure distance. Now I've formatted the workouts by distance, but you can easily convert them to time by multiplying 6-7 minutes per mile. The LI program builds towards completing your best 4 x 1 mile and best 3 x 1.5 mile workouts. A single 5K would also be a good workout for this category (think of it as a single long interval), and you might participate in a local race to give yourself some focus, to get used to running in a group, and practice dealing with competition anxiety.

Before performing an LI workout, be sure to warm up thoroughly. This can't be stressed enough. Warming-up for interval training needs to be longer and harder than most people tend to do. The recovery between intervals, if running more than one interval, should be adequate to maintain intensity for the next interval. As always, include some active recovery (easy jogging); don't just collapse in a heap on the ground. The actual amount of time can vary depending on how far/fast you run, but in general if you are doing mile repeats, recover for the same amount of time it took you to run (1:1 recovery-work ratio). For longer intervals, that same recovery period will probably be adequate and the recovery-work ratio will be a little smaller, but if you need a little extra time, take it. Again, the critical point is to keep up the quality of work intervals, not obsess about the amount of recovery.

The prescriptions for SI and LSD are pretty specific, but the Long Interval format is more flexible and variable. The key is spending time at speeds a little slower than your Physical Screening Test 1.5-mile pace while covering more than 1.5 miles total distance. The optimal paces, interval lengths, and total distances covered are hard to prescribe exactly. It's a bit of a grey area, and a number of different formats will provide benefit. These workouts are similar to what many coaches would call “threshold” or “tempo” workouts. Workouts can be formatted by time or distance, depending on which is more convenient or psychologically gets you to work harder.

Workout example for Long Intervals:

  1. 15-20 minutes continuous (one long interval)
  2. 5K continuous (one long interval)
  3. 2-3 x 10-15 minutes (30 minutes max)
  4. 2-3 x 1.5-2 miles (4.5 miles max)
  5. 3-4 x 6-7 minutes
  6. 3-4 x 1 miles

Long Interval sessions involve moving for as much as 15-20 minutes without stopping at a pace approximately 90-95 percent of the maximal pace you could hold for that duration. The workout should be very demanding but not totally exhausting. On a scale of 1-10, with 10 being the greatest effort possible, the workout should feel like 8-9.

Long Interval pace example:

If you're 1.5-mile time is 9:00 (6 minutes per mile), do mile repeats around 6:30 per mile. I emphasizes this as just a ballpark figure. As a general guideline, keep it simple: go pretty hard, but not absolutely all-out.

A good place to start with LI workouts is 2 x 1 mile. The PTG provides a sample schedule that builds up to a total work distance of either 4 or 4.5 miles. Interval lengths include 1, 1.25, 1.5, 2, and 3 miles. Other distances could be used, and time could be substituted for distance, estimating 6-7 minutes per mile according to the schedule.

When performing more than one repetition for a LI workout, allow yourself sufficient recovery between repetitions so you can maintain the desired intensity of 90-95 percent of maximal pace. A reasonable recovery period is approximately half of the work time for the longest intervals and about 1:1 work-recovery for the shortest intervals such as one mile. This should work out to 7-10 minutes of recovery after each interval, but don’t worry too much about the exact time. During the recovery period, spend most of the time in active recovery, moving at a low intensity - slow jog, or brisk walk.

As with all things related to your physical preparation, it's a good idea to start conservatively and build the volume for this category of workout slowly. Start with 2 miles of work; this might be 2 x 1 mile, or a single 2-mile run. As your fitness improves, increase the total distance using 1-4 intervals of 1-3 miles in length until your total work distance becomes 4-4.5 miles (about 25-30 minutes). The PTG provides an example of how this can be accomplished. If you become very fit and stick with the program beyond 26 weeks (it is important to remember the structure of the PTG can be followed indefinitely, not just for 26 weeks), don’t exceed the total mileage. Work on getting faster over the same distances. Be creative, try different formats, have fun, but stay within the general parameters. Use at least one format (such as mile repeats) on a regular basis so you can compare apples to apples as you chart your progress.


Finally, but most importantly, let's consider the right pace or intensity for your LI workouts. The PTG says, ‘These intervals typically involve moving for 7-20 minutes without stopping at a pace approximately 90-95% of the maximal pace you could hold for that duration. The workout should be very demanding but not totally exhausting. On a scale of 1-10, the workout should feel like 8-9.’

In regards to pace it's not an all-out sprint, because you couldn't hold that pace for even one minute, let alone 2 miles. It's a hard session, but not absolutely all-out. If it's your first time, it's okay to be a little conservative and maybe only go at 80-85% of what you think your max would be for 2 miles. After you try it, you'll have a better idea. Then next time go a little harder, until you find the pace you know is almost but not quite your max. I like to say that after a LI-type workout, you know that if somebody had put a gun to your head you could've gone a little faster, but that's how much extra motivation you would have needed.

In terms of actual pace in reference to a 1.5-mile run time, a ballpark figure would be to start out doing mile repeats about 30 seconds slower per mile than your 1.5-mile pace


For LSD workouts, focus more on duration than intensity. If you're exceptionally fit, you might perform 90 minutes of continuous movement in one session, though starting closer to 30-40 minutes is more realistic. A practical goal to prepare for BUD/S or BCS is to build up to comfortably running 10 miles without stopping. The sample schedules in the PTG call for two LSD sessions per week, beginning with only a few miles initially. If you're beginning the PTG as an experienced runner with a solid training foundation, you may begin with more mileage. Be sure to use good judgment; don’t overtrain at the start of your preparation.

The use of the term 'LSD' may be confusing or misleading for some. While it does stand for Long Slow Distance, it only means 'slow' relative to the paces you use for interval workouts. The pace has to be slow enough to allow you to complete the required distance. But 'relatively easy and relaxed' doesn't mean 'no effort' and it doesn't mean that you won't sweat. A certain amount of intensity is required before adaptations will occur. Rather than being 'slow' or 'easy', LSD training can actually be quite challenging, and I prefer to think of LSD as Long Steady or Sustained Distance.

Figuring out the right training intensity

A simple and accurate way to determine the intensity for LSD is to use the "Talk Test". Pay attention to your breathing, and choose a pace that causes you to breathe somewhat hard but not too hard. You should be able to talk in short/choppy sentences, not gasping uncontrollably but not making longwinded speeches either. Some people prefer to monitor their heart rate during training, which requires laboratory stress testing for complete accuracy, but I believe monitoring your breathing is simpler and just as effective for gauging the right pace to enhance endurance without overextending yourself. As your fitness increases, you're able to work at a greater percentage of your max for extended periods without gasping for air, so the Talk Test is self-adjusting relative to your ability.

The impact of endurance training

Endurance training may have some slight negative impact on strength and speed. As the muscles receive multiple signals from various training stimuli (LSD workouts, lifting heavy weights, sprints) some of the signals will conflict with each other. An Olympic weightlifter or hurdler trying to maximize muscle mass or contraction velocity would want to significantly limit LSD training. But elite athletes operate in an arena where specialization is required, and the difference between winning and losing is just a tiny fraction of a percent of actual performance. For BUD/S or BCS students and potential SEAL or SWCC Operators, it's essential to be well-rounded and capable across the entire physiological spectrum. This means being able to move effectively in short bursts and lift heavy weights, but also able to cover long distances over rough terrain and still have a steady enough pulse to shoot straight when you reach the target. It also means having the ability to recover quickly after a max effort to be able to go again with little down time. Training for such diverse situations requires a little give-and-take, and optimal adaptations won't occur across the board. But realistically, adding a couple tenths of a second to your 40-yard dash while knocking five minutes off your 10K run is a good trade any day.

One common argument against LSD training is that high-intensity interval training is sufficient to substantially increase aerobic capacity (VO2max). This is true and one of the reasons I include interval training in a general fitness program, but a large aerobic capacity is not enough to guarantee ample endurance. The Three Factors of Endurance are aerobic capacity (VO2max), lactate threshold, and economy (the rate of oxygen consumption required to hold a given pace; related to efficiency).

Economy is affected by body type (body mass, height, limb length) as well as technique. You can't alter your genetics, but both interval and LSD training can lead to better technique and improved economy (and therefor greater endurance). A large aerobic capacity requires a big strong heart that can pump many liters of oxygen-carrying blood per minute, and the intensity of interval training will stimulate the pumping chambers of the heart to become larger and more powerful, resulting in increased stroke volume and cardiac output. A high lactate threshold allows you to work at a greater percentage of your max for prolonged periods, and to recover more rapidly between shorter, high- intensity bursts. I like to include both interval and LSD formats in BUD/S and BCS training because they complement each other. The ability to recover more quickly/completely between intervals as a result of LSD training allows you to work harder when doing 400m repeats, for example. The prolonged, continuous nature of LSD training stimulates increased muscle capillary and mitochondria density, resulting in an increased ability to optimize blood flow, remove excess heat and metabolic waste products, and utilizes fatty acids for fuel, all leading to increased endurance and more rapid recovery. The useful adaptations stimulated by proper LSD training are not adequately addressed solely by Interval training. Limiting your fitness program to short, high-intensity workouts without corresponding LSD training is like building an automobile with a high- performance, large horsepower engine but a tiny 5-gallon gas tank. You may go fast but you won't go far.


The PTG running schedule includes two LSD workouts per week. I'd recommend an unbalanced format that includes a longer session as well as a shorter session (about 50-75% of the distance of the longer session). The pace would be approximately the same for each session; don't do the shorter run at too fast a pace and turn it into a LI workout. A general target would be to build up to a longer run of 10 miles with a shorter run of 5-6 miles. (Note: build up means a gradual increase over time. For example, during Week 13 you might do a long run of 7 miles and a short run of 3-4 miles, adding 1/4-1/2 mile to each session each week.)

Some experienced, competitive runners may have the background and ability to eventually go even longer than 10 miles, maybe as far as 15 miles or more in a single session. If you have the desire and ability and necessary baseline preconditioning for that kind of mileage, I wouldn't discourage you. But I've no basis for claiming that being able to run comfortably for more than 10 miles will be a significant advantage, so I don't want anyone to feel they absolutely must exceed that distance before starting BUD/S or BCS. It's possible to eventually add more weekly LSD sessions (I strongly recommend limiting Interval training to one SI and one LI session per week), but follow guidelines for increasing mileage no more than 10% per week. Running five or more days per week might not be a problem if running is your only training, but make sure it doesn’t compromise your swim training or strength training.

As you become more experienced as a runner, consider entering a 10K race to work on performing in a more competitive environment. Be sure to incorporate general as well as specific injury-prevention strength training as well as flexibility exercises into your running routine. Review the recommendations in the PTG.

Vary your running surfaces

For LSD, an important strategy is to vary the terrain regularly. Run on flat, hills, pavement, dirt, grass, or sand (even snow). Plan a route that includes changes in terrain. Exposure to different surfaces will enhance your overall conditioning and ability to perform in different environments. All may be beneficial, but none are best, and too much of any of them is probably bad.


I can never repeat enough the importance of taking a long-term approach, and to encourage SEAL and SWCC candidates to focus on achieving consistent gradual progress over several months, rather than trying to achieve instant gratification and stupendous results right now (because this rapid initial increase is almost certain to precede an early plateau followed by a decline in performance).

Week 1 of the PTG's 26-week program begins with four 400m (1/4-mile) intervals, at a pace only slightly faster than your recorded pace for a 1.5-mile run. Even if you're an experienced runner and are able to begin following the PTG, running more LSD miles than are listed for Week 1, I still encourage you to begin the SI workouts at Week 1 and perform only four repetitions. You should initially shoot for a pace during your 400m repeats that's about 4 seconds quicker than your 1/4-mile pace during your 1.5- mile run and no faster.

A session of four repeats at this pace is probably not going to feel too tough, but that shouldn't concern you at this point. You just want to set yourself up for long-term improvement; eventually you'll achieve some real speed. I've talked with SEAL and SWCC candidates who've become burned out or suffered from complaints such as lower leg pain, and this almost always correlates with doing too many intervals at too fast a pace too early in training. Some coaches would recommend not using intervals at all in the early weeks of training, but I feel it's safe and will lead to better long term performance as long as the prescribed guidelines are followed.

Build up and warm up to 8 interval runs

Since the SI sessions for the first few weeks will be relatively short, use the time to establish a thorough and effective warm-up routine. Add another interval to your workout every couple weeks according to the schedule in the PTG. Don't worry about increasing your speed until you build up to the full eight intervals. Work on consistency and pacing, trying to run each interval with no more than one or two seconds difference between your slowest and fastest. After you build up to eight intervals, doing them all consistently near your goal pace, you can start working on doing them a little faster every week. Still, don't be in too much of a hurry to go too fast too soon, and keep it steady. It defeats the purpose of the workout if you do the first interval in 65 seconds but expend all your energy, get slower and slower with each repeat and finish with a 2-minute interval. Between intervals, take enough time to adequately recover (about 2 to 2.5 times as long as it took to run the previous interval). Stay on your feet, keep moving, do a little jogging for active recovery, and stretch as needed.

Train smart by recording your stats

Keep a record of all your sessions. After developing a fairly consistent pace with a steady rhythm, every week try to knock another second or two off your last couple intervals, finishing each workout strong. For example, if the previous week you finished eight 400m intervals at an average pace of 1:30, continue to hold a 1:30 pace for the first few intervals of your next workout, then bring it down to the 1:29-1:28 range for a couple intervals, and then open it up for the last rep. The week after that, start the SI workout at roughly the same pace as your average from the last session and repeat the process all over again. If you're patient, you'll eventually see a significant reduction in your times. Taking even one second off your average pace each week for fourteen weeks adds up to a pretty big improvement. If you train smart, you'll develop enough experience to be able to push yourself hard from the start but still have enough stamina to finish the whole workout. As training becomes more physically and mentally demanding and it becomes harder to get faster from week to week, you'll improve and refine your tactics for dealing with the discomfort and the urge to quit. This application of determination, strategy and goal-setting to achieve a long-term objective using short and medium-term targets is consistent with the type of mental toughness required to get through BUD/S or BCS.

Use variety to shake things up

For variety, rather than always doing 400m repeats, you can create alternate formats of SI sessions using 200m-800m intervals as well as the standard 400m. For example, use an occasional 600m or 800m to work on sustaining your 400m speed a little longer. Another example might be 3 x 400m, followed by 4 x 300m, followed by 4 x 200m, which will let you keep your speed up even more. Use any combination that adds up to no more than 3200m (approximately 2 miles). It probably won't be possible to hold your 400m speed for 600m or 800m repeats, so expect to go a little slower, and obviously you would need more time to recover after 800m compared to 400m. On the other hand, for 200-300m you'd be able to go even faster than 400m speed and take a little less recovery. These variations will let you work on different parts of the speed continuum as well as shake things up mentally. Make sure to do your SI sessions on a firm even surface that's safe (you don't want to sprain an ankle or twist a knee) and fast (after all, it's a speed workout). Trail running or the beach is good for some LSD work but not optimal for SI. A good outdoor track is ideal, but a bike path or similar surface can work (watch out for traffic). Be careful if you use an indoor track, because if it isn't banked properly the tight turns will be hard on your legs. If you're temporarily unable to run for a SI workout (minor injury, bad weather, etc.) follow my general training recommendation (also applies to LSD) and use an alternate activity to mimic the intensity of the regular workout. Use a treadmill with a slight incline, a stationary bike, or a rowing machine to work as hard as you can for the same time periods it would take you to run your 400m repeats and spend the usual amount of time recovering.

Finally, remember to put SI sessions in the proper context of total training. Interval training is a fantastic tool to improve fitness, but it must be balanced with lower intensity sessions. The 'all-out, all the time' approach is counterproductive. That's just a matter of human physiology, and we can't change it, no matter how tough or hardcore we think we are.

For anyone interested in a (fairly technical) summary of research that has investigated the balance of training intensity and duration for high-performance endurance athletes, please see the following: Intervals, Thresholds, and Long Slow Distance: The Role of Intensity and Duration in Endurance Training -

From the article's conclusions:
'Currently, there is great interest in high-intensity, short-duration interval training programs. However, careful evaluation of both available research and the training methods of successful endurance athletes suggests that we should be cautious not to over-prescribe high-intensity interval training or exhort the advantages of intensity over duration.'

'HIT should be a part of the training program of all exercisers and endurance athletes. However, about two training sessions per week using this modality seems to be sufficient for achieving performance gains without inducing excessive stress.'

'The effects of HIT on physiology and performance are fairly rapid, but rapid plateau effects are seen as well. To avoid premature stagnation and ensure long-term development, training volume should increase systematically as well.' "