In order to be really honestly "fit," your heart and your cardiovascular system have to be ready for anything. You might notice that if you workout exactly the same way every day, and then you suddenly try something new, you might suddenly feel really out of shape when you didn't before. But throwing sudden bursts of intense exercise into an otherwise "routine" routine can really improve your heart's ability to respond to sudden demands, which will improve your overall fitness and get you to the results you're looking for that much faster.

And did you know (this seems like an outrageously high percentage!) that high intensity interval exercise like this can reduce the risk of heart disease by as much as 100% compared to those who only work on endurance? Wow.