One must understand that there are two ways how energy is produced in the muscles etc. I don't really remember the correct terms but you get the picture. There is aerobic meaning work with oxygen and then there is anaerobic and that means more or less without oxygen. Fats need oxygen so that it can be used as energy. The process of fat burning is also quite effective as you get a lot of power but the process itself is slow process as it takes more steps than carbon metabolism. Carbs on the other hand can be used as energy without oxygen, that is anaerobic, and the procedure to convert carbs into the energy muscles and cells can use is faster.
This will lead to the conclusion that if one is training endurance with low level, meaning all or most of it is aerobic, that can be done with fats only. As the training gets harder, meaning more or less more anaerobic, the energy will be taken from the sugar reserves that are stored in the muscles and other organs. The fact is that when the excercise is hard and long enough (it can be actually relatively short if it's hard enough), the sugar reserve will run out and after this it's only possible to work in the aerobic level. Ok, this leads to the question: how do you want to train and for what purpose and how much?
Fat burning process can be made more efficient and "faster" but it will NEVER be as fast as using internal sugar reserve, no matter how much one is trying and it can never be made to work in unaerobic mode. What happens that if one gets in better shape (not meaning lean but that oxygen intake gets better) , one can work harder and still staying in the aerobic area (compared to the old fitness level).