This is my take too. Maybe you can fine tune a little to reflect that some folks may be better able to store glycogen than others, and maybe some folks are more efficient than others, but being able to run about 2 hours on the amount of glycogen you can store from carbohydrates is a pretty good rule of thumb. The faster you are, the more ground you can cover in those 2 hours, and for some people it may be more than 2 hours; but even the elites take in some carbohydrates (in the form of drinks) these days.Once your glycogen is depleted, you go to burning fat and muscle, and as a result, your brain goes wonky because it doesn't work properly with those sources of energy.
Lrc phd wrote:
You only have enough stored glycogen to make it about 20 miles. So if you want to run fast, yes.