Interesting insight. If you were on a 500 calorie diet and "bonked" while cycling then presumably it was shortly after a meal, since at that level of calorie restriction you're going to be running mostly on fat reserves directly rather than sugar via glycogen. It is absolutely possible to run marathons without having eating a gram of sugar. The advantage is precisely that you can't hit a wall when the glycogen runs out. But apparently the keto diet is not quite as performant at the margin, which is why athletes haven't all switched to it. It does deal with the hunger issue though, by all accounts.