That's another thought, how long before you deplete fat stores? It seems to me that the body would struggle trying to keep the fat burning going after a bunch of exercise before...
monitor how many calories you are getting in right now for a few weeks. Not losing fat? Then consider lowering the cals or working out harder and/or longer. Not getting bigger? The consider...
How much protein is used for energy during continuous exercise? I was thinking somewhere in the range of 5-15% out of total calories burned but have had no luck finding concrete studies...