I hope you will forgive me for this long post. During rest, the body use slightly more fat than carbohydrate for its fuel, along with a small percentage of protein. In the first fractions of seconds of exercise, muscles depend on ATP for muscle contraction. PC is also produced from fat and glucose. A lot of heat is released in this production.
Aerobic metabolism happens inside the mitochondria of the cell.
Both fat and glucose are metabolized aerobicly in the mitochondira of the cell. This can only last for short time because the glucose is changed to lactic acid and held as such until there is enough oxygen. In this state the muscle are said to be in a oxygen dept. Lactic acid buildup can cause burning pain and can lead to exhaustion within seconds if the blood cannot clear it away. Therefore you have to stop or reduce the exercise after a while and your body begins producing energy aerobically again.