Your muscles and brain run on glycogen. More importantly for this question, Glycogen fuels your muscles. When you exercise with high intensity for 20 minutes or more this is when your body switches from a primarily carbohydrate based metabolism to a fat metabolism, as duration increases so does amount of fat burned for fuel.
SO IN ADDITION
I suppose high-intensity would be > 70% V02 max Carbs are used as primary fuel here, if low-intensity or under < 30% V02 max, fats are primary fuel.
So if the total energy expenditure is relatively high, 50% from carbohydrates, 40% from fat, and 10% from muscle, which creates lactic acid.
If your prolong your exercise, you shift from carbohydrate dominant metabolizing toward fat metabolism. This is due to an increase in breakdown of triglycerides (cholesterol) or (glycerol + free-fatty-acids).
So in reference to the muscle that is used, it actually comes from the glycogen, as the primary source of carbs (keep in mind you're also burning fat after 20min) for the first hour or so.
85% V02 max and you're burning 60% muscle glycogen, 15% glucose, 20% FFA, <5% muscle triglycerides
Fat burns in the flame of carbohydrates