How much fat is burnt compared to carbs/protein during biking/running/cardio exercise?

If I were to do some type of cardio, like running, biking, or swimming, and burn a total of 600 calories in one hour, how many of them would be from fat, carbs, and protein, on average? Assuming I'm 150 lbs and 6'0", with about 10% body fat, could be a couple percent higher than that though.

Also, would the answer change if I had eaten a meal full of carbs beforehand? Thanks.

When you exercise, your body first looks to the glucose from the carbohydrates you've eaten for energy. Once that's all burned up, your body will tap into it's fat stores. So if you're looking to burn fat, the best time to exercise is in the morning (the glucose was burned up from 8 hours of fasting).

If you're not a morning exerciser, then you can expect for the first 30 - 35 minutes of physical exertion your body will be burning up the carbs you ate. After that, it goes for the fat. Protein is used to build muscle, so your body will not technically "burn protein" unless it has to start eating muscle for energy. That is, of course, not a good place to be.

If you're gonna be doing a long endurance event, something like a triathlon (I'm a triathlete so I'm confident in the advice I give you) ... for such an event, you'd want your breakfast to be a good mix of protein and carbs, like a peanut-butter and banana sandwich. During the race, you'd want to take a Gu or two, to give your body and instant surge of energy from carbs.