Q: How much rest should you take between intervals?
A: I put a lot of thought into the interval portion of your workouts--how far, how many, how fast. The recovery or rest portion of your workout may be merely an afterthought to you, but, perhaps you should consider it more carefully.
Rest is an important variable to take into account when designing challenging, race-specific workouts. Manipulating the amount of rest in a given workout changes the benefits gained and physiological systems targeted. Tweaking the time taken between intervals can greatly modify the overall intensity of a workout, so you need to put as much thought into the rest between your intervals as you do the hard running.
In general, shorter rest periods ramp up the intensity of a workout because recovery is incomplete and your heart rate remains high when the next interval begins. Typically, no matter what distance you are training for, you use short rest to keep quality up in a workout and your heart rate high. In addition, short rests can be utilized to break up a tempo workout and allow an athlete to run a fair amount of volume yet maintain intensity.
I adivize using longer rests in workouts consisting either of very long intervals or very demanding intervals. Longer rest periods when used training for shorter races are most beneficial so you can run a repetition almost flat out, then recover fully before you begin the remainder of the workout. Quality is the focus here, not quantity. In general the more taxing the interval is, the faster it is, the more recovery called for.