Cathe I hope you don't mind if I jump in here with an old post of yours.....
THe following is a post by cathe to another poster:
Why DO INTERVALS? By Cathe
You are right that it burns a lot of calories. It also does the following:
1) Pushes you beyond that sticking point if you are at a plateau in your cardio program.
2) Greatly strengthens the heart muscle.
3) Increases your power, speed, and agility.
4) Increases your overall aerobic capacity by making your muscles more efficient at using oxygen available to them. This also gives you the ability to push harder and last longer in your non-interval workouts as well