So if you were doing cardio for an hour or two, your muscles should pretty safe even at high intensity, as long as you have been eating enough for your glycogen stores to be sufficient.
Cardio for an hour or TWO??? I think that is insane if your goal is to burn fat while retaining lean muscle.
In my opinion, going above your upper limit for more than a few minutes at a time is counterproductive to the goal of fat burning, because yes, your body will burn muscle, NOT fat. I find that many people think doing cardio for hours at a time -- at an intensity way above their upper limit -- will help them achieve their fat loss goals, but I think it actually hinders that goal -- not helps it.
Now note that I am talking about the goal of burning fat while retaining lean muscle. If your goal is to simply build endurance, then that may be a different matter in terms of what works and what doesnt.
I would love to hear Kollisions opinion on this though since he is the certified personal trainer around here.
Nat, you are totally right. The body will use up the glycogen stores and then move onto muscle if there is no more glycogen to be used. Politically correctly speaking, Cardio would be at higher intensities than Fat Burning, it is two different things.