I think the trick is to come 'close' to hitting the wall and then refuel to recover, rest to give your system a chance to improve… or you might find yourself overtraining… it's too easy to incur injuries if you truly 'hit the wall'…
I had saved this part of an article…(OT= over training)
Neuroendocrine effects of over-training syndrome in endurance athletes. (Phytotherapy Review & Commentary).
Townsend Letter for Doctors and Patients; 7/1/2003; Grant, Andrew
The simple model is two or more subsequent bouts of hypoglycemia that lead to OT. This model is antecedent hypoglycemia with subsequent hypoglycemia. An athlete can induce autonomic failure by two or more subsequent bouts of endurance training lasting longer than 90 minutes without the use of a carbohydrate source. Ninety minutes of exercise is required to deplete glycogen stores, which will in turn cause hypoglycemia. This simple model can be exacerbated by a low carbohydrate diet, which does not adequately replenish glycogen stores between consecutive days of training. This is the simplest pathogenesis of OT in endurance athletes.