The word interval means repetition in respect to track athletics. Interval training, then, means repetition training, in which specific distances are run over and over again. An interval usually is run at a brisk pace. In order to recover before starting the next interval, a short or long recovery phase is requires. Recovery may be in the form of a slow jog, a walk, or a rest, or a combination of all these. Favorite distances range from 110 yards to one mile, but 440 yards tends to dominate.
Runner's World Magazine
Costes, Nick, "Interval Training" (1972). Prairie Striders Library Collection. 224.