I think his starting point is the amount of glycogen in your liver and muscles before you start the race. Of course, if you really want to push it, your storage needs to get replenished. I listened the last part in the car radio, a MIT student 100 lbs, 65 resting heart rate, can run 2 hours, 44 minutes, if she had enough glycogen in her system at the start. He said 1,900 of carbs (calories from carbs??) the night before the race.
If we take even a portion of a GU at the 10 mile mark and the 15, 20, maybe you'll have fresh legs again. That way the store of glycogen won't ever get near empty.