You only need glycogen for that short burst--so much of the glucose broken down during exercise isn't entirely oxidized, just broken down so far as pyruvic and lactic acids, which can then be used to replenish glycogen stores through gluconeogenesis--so short of insane, bootcamp/Biggest Loser levels of intensity, glucose requirements with training don't go up nearly as far as you'd expect them to. I have no trouble sustaining high volume/high intensity training, in fact I feel better when doing it.

I did a three-day water fast last fall. I've done water fasts before, and taken advice to be sedentary, rested. Always felt like crap. But if I sit on my butt for three days straight, when I'm not on a fast, I end up feeling like crap anyways. This time I did my usual workout (I do full body, every day, same workout). My workouts got better every day, rather than worse. It was also the best-tolerated water fast I've ever done.