combat_action - don't be fooled into things like cardio eating away at your fat and muscle stores. It's simple math. If you eat an excess of calories by the end of the day, and are getting adequate protein (one gram per pound), you're not wasting away that muscle. Unless you're at an extremely low bodyfat level already, training on completely empty, and are doing it at a high intensity, for an extended session, and often, the muscle won't dissipate so easily.
I do agree with Ghetto in that, sprinting is the way to go. It's intense, but utilized properly, you end up with mass added to your legs, explosive drive in them, more calories burned (if this is your goal), and do it all in a shorter time than longer duration cardio.
Of course, it's all relative to your goals. If you want to be explosive and powerful, sprinting will do it. If you're in the military, forget about sprinting. Go for longer distance cardio. If you have no real direction to do, there's no reason you can't do a bit of both. Just know that, so long as you have a caloric excess and adequate protein, along with resistance training I might add, you won't lose that muscle so easily.
Then again, this is a heavily debated topic and there's always someone who will agree or disagree with the points I've made. The best you can do is look at your specific goals now, and in the far future, and take it from there.