It simply is not possible to burn fat by eating less during a workout. Only by putting in long miles and eating intelligently is it possible to become more efficient at burning fat. Glucose is also the only fuel the brain can use. Training in a glucose-poor state makes it difficult to think clearly and is a real safety risk. Though trained athletes are generally able to judge how much to drink, large fluctuations in need can occur due to fatigue, temperature, altitude, humidity, air flow, fitness, food and the type of fluid consumed.
Dehydration can be deceptive because it feels exactly like fatigue.
Late in a hard ride, when you feel tired, it may simply a case of dehydration. Don't sell yourself short, drink up before getting tired. Dehydration impairs thermoregulation through reduced sweating and blood circulation. As dehydrated blood gets thicker it creates cardiac stress which impairs the metabolic processes necessary for physical effort. Losing more than a few pounds on a ride, or more than a couple of pounds overnight, is a good indicator that you didn't drink enough.