You will not get a sugar crash from complex carbs. Even glucose won't give you a crash if taken just before or during intense sustained exercise. Burning protein creates ammonia and urea which the body then has to filter from the blood, it also uses a lot of water. Fat requires the most oxygen to burn.

Most of the energy used, is from glycogen stored right in the muscles themselves.(At full capacity: about one pound of sugar hooked to 4 pounds of water or about 1800calories) Blood sugar(glucose) is used much more slowly as the body likes to save it for the brain, which burns nothing but glucose.

Fat is used from various stores in the body makes a large portion of the low and slow metabolism and can be eaten at almost any time.

Just after exercise one should immediately eat a moderate sized low fat meal of simple carbs and some protein to restore the glycogen while the muscles are most receptive, to halt muscle breakdown (they will try to eat themselves, silly things) and provide new building materials.

This should be a low fat meal because fat will slow up the absorption of the carbs and protein(which should be in an easily digested form. well cooked, ground, and mashed, taken with water.)

An hour later follow up with a more balanced meal of complex carbs, fiber, protein, and fat