As oxygen becomes limiting, sugar burning takes precedence. Fat burning actually declines relative to the earlier pace since more TCA cycle energy generation is utilized by carbs, rather than fats. With this system, only a little of the potential energy of glucose is released, but it is done so in a very rapid manner to meet short duration, very high energy production rates. The limitations to the anaerobic system is that lactic acid is produced as a result and lactate levels reduce the pH of working muscle to the point that the enzyme systems don't function well. Maximum duration is on the order of 6 - 15 minutes depending on the exact muscle groups and the starting levels of glycogen in the working muscles.
Aerobic conditioning serves to promote a greater reliance on fat burning as the fuel choice.
After training, the same intensity level can be performed with a greater percentage of energy derived from fat burning which preserves glycogen stores in that muscle. Eating just prior to a duration event results in a shift towards more carb burning. This is simply a consequence of increased levels of circulating levels of insulin. Carbs taken during an event will blunt the release of free fatty acids from peripheral fat cells, and are likely to shift energy production inside the muscle cell toward more carb burning. This discussion should in no way imply that low intensity exercise is the best form for weight loss.