I assume that was directed toward me?

Glycogen is something that nobody else had mentioned in the thread, which is why I brought it up. Is it the main energy source for endurance sport? No. Is it a significant energy source? Yes.

As with most things biological, wasted energy leads to lower levels of 'fitness'. The skeletal muscle tissue synthesizes glycogen while we sleep. It produces only as much as it thinks it needs based on your general level of activity (let's call it a bookmark, or benchmark). I'm making up these numbers, but let's say your average use of glycogen each day from your routine activities is 100g. Then one day out of the month you go and do an endurance sport and ask your body to give you 1000g of glycogen over the course of the event. You will hit a wall, no matter what nutrition regiment you decided to use before/during/after your event.

Now, on the other hand, if you use 1000g of glycogen every 2nd day in your routine, your body will provide the stores that support your needs. If you don't use it your body will (over time) go back to producing less and less until it reaches equilibrium with what you actually use. This is a natural conservation of energy/resources, and yes, the muscles don't "think" it's controlled by a complex feedback system of various chemical pathways that we usually just call 'homoeostasis'.

Here's a cool diagram/graph too:
Screenshot.jpg