HIT (High Intensity Training) is pushing your body to it's limits in short spurts. It increases muscle mass faster than endurance training, and it channels fat into your bloodstream to be used as energy quicker. I started doing this because longer runs became taxing on my knees.

Think 30 seconds or more- whatever you can take. (If you are able to run at that speed/intensity for too long, however, it might not be hard enough for you.) Rest for a moment or so, catch your breath. Take a breather. Get back in it. Keep going. Push yourself further. 30 min 3/4 times a week should do it.

Nothing better than a half hour run to get adrenaline going....