As a former biochemist, I could write pages, but the basic paradox is that your body wants to burn glucose/glycogen first, that's what it does best, and that's what your body needs to do for you to perform at your best athletically. However, to burn fat, you must be mostly depleted of glucose/glycogen. This is why, while dieting, don't count on any personal bests athletically. It is perfectly natural for you to bonk more easily while losing weight. To prepare properly for a hard ride, you need to eat well for 3 days before the event. It takes about 3 days to fully replenish the glycogen stores in your liver and muscle. However, while you are doing that, you are not burning fat. This is the dilemma. The proper way to prepare nutritionally for peak performance is pretty much the opposite of what you do to lose weight.