As race fitness improves, the speed of the intervals should keep pace but should not exceed the race improvement level. Instead, it is better to add more hard work e.g., add another mile repeat rather than increasing the pace faster than the improving 10K and 5K average pace. This added hard work will add more fitness (better endurance and anaerobic threshold development) than will doing less hard work at higher speeds.
Length of Total Interval Session
The total length of the hard effort in an interval session should be in the range of 2 to 5 miles. It will tend toward the lower amount with shorter intervals and toward the upper limit for longer intervals. Most runners find 3 or 4 miles of hard effort to be the right amount of quality work in a single session. For example, typical workouts are: 4 x 1-mile; 12 to 16 x 400m; or 20 x 200m.
If the runner finds that the speed of his intervals is at the maximum recommended speed (see section above) and he is reaching the 5 miles of hard work and is not improving his times, one of two things are possible. First, if everything is being done right, he is in peak condition. Alternatively, the runner is turning into a workout king and needs to add more endurance training to be able to improve his race times.
The length of the workout will vary depending on other factors in the athlete's training. The greater the number of interval workouts, the fewer intervals needed in each workout. The higher the quality of the distance mileage, the smaller the amount of interval distance needed.
How Often to Do Interval Sessions
Finally, the runner needs to determine how often to do interval sessions. This will depend on the time of season, how well the distance running has gone, and the runner's physiology. For a slow-twitch athlete competing every other week and with a solid base of quality distance running, one long interval session and one short interval session during the fortnight should be sufficient, together with at least one other tempo run of 4 to 8 miles and several medium to hard distance runs.
At the other end of the spectrum, a fast-twitch runner who runs his distance mileage easy will need far more interval work during the same time period. Such an athlete might need two long interval sessions, two short interval sessions, and two tempo runs during a two-week period.
Athletes who race more frequently need to reduce their interval training sessions to accommodate the greater number of races, which are also a form of quality training. If there are fewer races, an interval session can be added. As racing season approaches, it is better to slightly under-train than risk overtraining, which causes fatigued muscles and requires substantial rest for recovery.
More: Are You Overtraining?
Interval training provides a level of fitness that is difficult for most runners to achieve via distance mileage alone. As with much of training, flexibility is needed in designing and performing interval sessions. Every runner is different and will react individually to different workouts. But if the purposes and principles of interval training are kept in mind, the runner should both enjoy and benefit from interval training.