Your body burns fat, cabs, and protein all at the same time, but not in equal amounts. When carbs or protein get burned, they produce 4 calories per gram used. When fat gets burned, it produces 9 calories per gram burned.
48 grams of fat can provide 432 calories of energy, if they happen to get used.
That's a very oversimplified theory of how the body works.
When you begin exercising, carbs are actually the preferred fuel source (ie: stored glycogen)-- especially during exercise that is more intense and uses the anaerobic metabolic pathways.
When glycogen depletes, the body will turn to burning fat as fuel, but it is not in the sense that you seem to think it is.
I suggest you do some research on the ATP-PC system, Glycolysis, and Krebs Cycle.