Much about muscle metabolism is clearly understood. Glucose is converted to ATP and other substrates to produce "energy".

That being known, the limiting factors become the ability to deliver and maintain glucose stores in the areas needed by muscles to perform work.

Of course, so many other factors influence glucose transport, enviroment, intensity of exercise, mechanics of exercise etc, that this will always be the "crux" of the matter.

So my question is, how do you predict glucose needs in a big ride if you do not know the"pace" or intensity you will be maintaining before hand? When does lack of fat or protein ingested become a limiting factor?