Aerobic Interval Training 101
Aerobic Interval are what many runners think of when they hear 'interval training'. These are often runs of 1-8 minutes at a high perceived effort, repeated so that the total mileage is around 3-6 miles. The longer intervals (5+ minutes) are often used with a recovery time of 1:1. The shorter intervals have disproportionally shorter recoveries (half the interval time or less). A measured distance is normally used, such as a track, but any reasonably flat surface you can mark with the distances works. (I use the local greenway, which is marked at quarter miles). Jack Daniels Running Formula refers to these as simply 'interval training' or 'I Pace intervals'.
1 Defining Aerobic Fitness
There are two useful measures of aerobic fitness for most runners. The first is how fast your body can use oxygen. This should make sense, as the body produces energy over extended periods by burning fuel, which consumes oxygen. This is termed 'aerobic exercise', literally 'with oxygen.' The measure of this fitness is called VO2max, which is maximum Volume of Oxygen (O2). The second measure is how efficiently you can run. If you can go faster for the same energy expenditure, you will be faster for a given aerobic fitness level. This is called vV̇O2max, for Velocity at V̇O2max. Most people can run at vVO2max for about 6 minutes. Improving either of V̇O2max or vV̇O2max will make you faster (or make a given pace feel easier).
2 Structure of Intervals
There are four variables for interval training; how fast you run them (intensity), how long your run for (duration), how long you recover for, and how many times your repeat the intervals.
3 How fast to run (intensity)
Intervals can be run at differing paces, which achieves different things. If you are not used to speed work, then starting off at a slower than optimum pace can reduce your chance of injury. The most effective pace is generally considered to be 1 to 8 minutes at 90 to 100% of vVO2max pace; however, that's only useful if you know your vV̇O2max pace! While you could have a VO2max test, there are easier options. One is to see how far you can run in 6 minutes and assume that is your vV̇O2max pace. However, Jack Daniels (the coach, not the distiller) has found that you can calculate an effective VO2max value based on your race performances. This number, which Jack Daniels calls VDOT can then be used to determine the speed for training runs, including interval training. There is an online calculator at VDOT Calculator though I would encourage you to buy Jack Daniels Running Formula
4 How long to run
Running for 1 to 8 minutes is a good general guideline. Common distances are quarter mile, half mile, three quarter mile and mile. (Or 400, 800, 1200 and 1600 meters if you are on a track.) There are times when shorter than quarter mile works, though these workouts tend to be focused on strength or maximum speed.
5 How long to Recover - Complete and incomplete recovery
For intervals lasting five minutes or more, the rule of thumb is to use a similar time for recovery (1:1). This can often be achieved by covering half the distance in the recovery, as you are likely to be running around half the speed. With these longer intervals, the aerobic stress on the body will build up over the first few minutes of the interval, with the maximum benefit obtained in the last part of the interval. For intervals lasting less than five minutes, the buildup of aerobic stress on the body will not have completed by the end of the interval. Therefore, the recovery time is shorter, so each interval starts without having recovered from the previous interval. This way the aerobic stress builds up over the first few intervals. Typically a recovery time of half the interval time is used.
6 How to recover
It is best to run slowly between intervals, rather than stop completely. The interval will produce Lactate, which is a natural part of exercise, not a bad thing. If you keep running in the recovery period, you will use the Lactate for energy and recover better than stopping.