Great insight from the New York Times:
The potency of interval training is nothing new. Many athletes have been straining through interval sessions once or twice a week along with their regular workout for years. But what researchers have been looking at recently is whether humans, like that second group of rats, can increase endurance with only a few minutes of strenuous exercise, instead of hours? Could it be that most of us are spending more time than we need to trying to get fit? The answer, a growing number of these sports scientists believe, may be yes. – from NYTimes
This has always worked for me.