It's been a while since I researched this, so don't take it as gospel, but ...
There are several stages in the sequence toward metabolic exhaustion:
When you first start to exercise, the muscles (other than the heart muscles) burn "blood sugar" -- glucose. This provides instant energy, so it's always circulating (until the supply is exhausted). Blood sugar can be replenished from the digestive system and the liver, but the digestive system tends to shut down when you exercise hard, and the liver cannot pump out blood sugar very rapidly.
So, after a few minutes of reasonably intense exercise, the muscles begin depending on glycogen. Most of the glycogen in the body is stored in the muscles, which is good, because when one, eg, lifts a 500 pound barbell the tension in the muscles cuts off the blood flow, so that the muscles are starved of glucose from the blood and must depend on their own glycogen stores. (There are several genetic disorders -- McArdle's syndrome, et al -- where the muscles cannot break down glycogen, leading to a number of symptoms, some rather dangerous.)
Some glycogen is also stored in the liver, and, when blood glucose levels drop, the liver will begin to convert this glycogen to glucose.
So much of the ability to exercise for extended periods is dependent on glycogen stored in the muscles, and when you deplete that glycogen you "hit a wall". But regular exercise, sufficient to burn most of your blood sugar and thus trigger the glycogen mechanisms, will tend to cause the body to store more glycogen in the muscles (in between exercise sessions).
When glycogen is depleted, the muscles will attempt to burn the last bits of glucose in the blood. But this is bad for the brain (which does not have its own glycogen stores), so the liver kicks into high gear and starts converting fat into ketones. The muscles and brain can "run" on ketones (and, interestingly, the heart can ONLY run on ketones), so you get an extra burst of energy.
But this burst of energy comes at a cost. The high levels of ketones produce headache, fatigue, dizziness, insomnia, and nausea. Acetone in the blood produces a sort of "fruity" smell to the breath. I can attest that it takes several days to recover from a major episode of this condition.