For longer activities there are some issues relying entirely on glycolysis. There are limits to available glucose in the liver and in the cell locally. All the energy systems are in concert, some leaned on more heavily at certain times.

You been reading your biochem like we talked about?

The body can run on ketones or glucose, but that does not mean it was designed to run on glucose. Do you think humans thrived most eating loads of fruit, like monkeys? Or did they adapt / evolve in a different way to become the apex predator? Have you considered what nutritional differences required infants to complete their growth outside of the womb? (Larger brain size required an earlier birth.)

It's more logical that humans were designed to run in a fasting state most of the time, with energy coming from stored fat (ketones). Food for early humans was not constantly abundant. Fruit is not in season for long, especially at more northern climates.

What are the foods of the kings and lords and what is the food of the peasants?