Fat and glycogen metabolism don't turn on or off at some time or level of exercise intensity. Instead, the ratio of fat to glycogen metabolism changes with exercise intensity
Yes, well put. I would just add the the ratio also changes with exercise duration as well. I wouldn't focus on trying to burn a higher ratio of fat. Just burn calories. Yes, if you do slow exercise you will burn a higher ratio of fat to glycogen, but at the higher intensities you burn more overall calories and end up burning at least the same amount of fat. Mix up your riding to include both the long slow burn and the high intensity rides.
but If Im metabolizing fat I don't care how much energy I have to ride.
Oh, yes you will! If you are truly out of your glycogen stores(aka Bonked), you cannot ride. Period. You won't feel a little tired. You will be more exhausted and incapable than ever.
The general rule of thumb is to eat 250 -300 calories an hour for the long rides. You will likely burn around 700-1000 calories an hour. If you don't eat, you will bonk. If you bonk, your ride will get cut short. You will burn fewer calories. You may even have catabolized muscle and will take days to recover before you can ride again = less burned calories for the whole week!
I have been riding to lose weight for quite a while. I sympathize with you. I know it goes against every fiber of your being to eat while exercising. We want to lose fast and eating seems counterintuitive. I assure you, you have to eat for those long rides. You will still lose plenty of weight anyway, and not eating will just backfire.