Amount of Rest
The interval of rest between hard efforts has historically been pegged as the time necessary for the heart rate to drop to about 120 beats per minute (bpm). This works fine for shorter intervals since it is typical that the rest period is about the same amount of time as the hard effort. However, for longer intervals, the heart rate will likely reduce to 120 bpm before the runner is physically ready to go again. Some runners simply jog one-half of the longer interval distance as a recovery period, and this takes about the same amount of time as the hard effort.
From this midpoint of recovery time, runners can vary the amount rest they take depending on their workout goal. Runners who are seeking to develop a greater amount of endurance can reduce their rest periods. Runners seeking to run faster during their hard efforts (usually to increase fast-twitch muscle development) can take longer rest periods.
Generally, within a reasonable margin of the midpoint recovery period (20 - 30 percent variation), it is simply a matter of preference to determine the amount of rest between hard efforts. The margin of fitness difference between a little more or a little less recovery period is inconsequential in the overall context of training.
The next question is whether to walk, jog or run during the recovery period. Again, this should be determined by the primary purpose of the training session. Walking obviously provides more rest than jogging if the rest periods are the same amount of time. Walking for recovery might make sense if the primary goal of the interval session is to maximize muscle development. If building endurance is the main goal, a walking recovery makes little sense.
Jogging during recovery is generally preferable to walking if for no other reason than the legs stay warm and loose between hard efforts, reducing the risk of injury during acceleration. Jogging during recovery also has the added benefit of keeping the runner running, which improves endurance and the mental toughness of the athlete.
Active running as "recovery" is used to build endurance. For example, a workout of 4 x 1-mile can have 1-mile recovery runs at 15-20 percent slower than the hard miles (e.g., if the hard mile is 6 minutes, the recovery mile would be 7 minutes). Prefontaine liked the workout that alternated 30- and 40-second 200 meters for 2.5 to 3 miles. This type of workout will improve anaerobic threshold but, like any fartlek, primarily serves to increase endurance. And at certain speeds, this type of workout is very tough and not for everyone.
More: 7 Ways to Avoid Overtraining
How Fast to Run Intervals
The speed at which a runner should do intervals is again influenced by the workout's purpose and the athlete's other training. A runner who does easy mileage is going to need to get more out of his interval sessions and will likely run intervals faster than a similar athlete who does higher quality distance running. In addition, speed is influenced by the volume of work to be done and must be consistent with the recovery time.
Going back to my example of doing 4 x 1-mile averaging 4:19, the reason the workout was not productive was because it was too close to my maximum ability. It simply broke me down too much, given the other training I was doing. Rather than being at race effort, the miles should have been done in the range of 80-85 percent of maximum heart rate, which would be about 10K race pace.
Pegging 1-mile intervals to 10k race pace is generally a good target for the typical runner. 800-meter intervals can be set at about 5K race pace. Intervals longer than 1 mile should be adjusted accordingly. If pace is being varied during the hard effort, the average pace is usually slowed. And for "cutdowns" the average pace is also slowed down slightly e.g., the first mile repeat would be slower than 10K pace and the last mile repeat would be closer to 5K race pace.
The pace of shorter intervals varies much more widely. If a large quantity of intervals are to be done, the pace will naturally be slower than if fewer intervals are to be done. If building endurance and anaerobic threshold is the main focus of the workout, the intervals should be slower than if developing muscles is the main goal. For building endurance and anaerobic threshold, the speed of a 400-meter interval should be slightly faster than 5K race pace. 300-meter and 200-meter intervals should be done at a slightly faster pace than 400-meter intervals.
With all interval workouts. it is important not to go too fast too soon. It is far better to start slowly and increase speed as the workout progresses than to do the opposite. To start the intervals at speeds that cannot be maintained reduces the quality of the workout and makes the recovery take longer. In addition, it is de-motivating to fail at a workout in this manner.