I can't say I personally use it, but most of friends who are really intense when it comes to working out or running in general use interval training instead. It definitely seems like the way to go, and it is ridiculously tiring.

I actually heard this on a health and fitness documentary a couple of days ago. They claimed you only need 3 minutes of interval training a day and it will have the same effect as the 2.5 hours a week you're normally supposed to spend exercising. Not sure I buy that...

But 50 minutes of it should definitely do the trick. I might even try it myself.

Interval training basically not only burns fat and builds stamina, it also trains your heart rate to stay at a normal level longer when you're running. What that means is that you'll be able to run longer and faster without burning out. Also, it doesn't lower muscle mass, like traditional cardio can.

I do interval training because that is how you improve your speed for cross country. As a cross country runner both interval training and long steady runs are hugely important.