I have been thinking about your post for most of the day. If you are an endurance athlete that eats a mostly paleo / low carb diet are you already on the low side pre-exercise?
I did a 2 hour ride with my sister on Saturday that turned into a little more of a hammer than originally planned as we rode out with a 20 knot tailwind. Oops! I definitely was at the "bottom of the tank" when we got back. I ate my normal meat, salad and small amount of carb meals for the remainder of the day. Sunday morning we did a 45 minute high intensity ride with a 20 minute tempo transition run. It ended up being a pretty tough brick. Afterwards I couldn't eat right away but when I could I was ravenous.
What made me think about this is how I was eating in the past when constantly training for triathlons. It was definitely more carb focused. Today I train 3 days a week endurance and I don't feel like I need the extra carbs except Sunday when I was so hungry I could have ate the paper plate my food was served on.
This also played into a question that I was going to ask Paul. On my heart rate monitor during the Saturday ride it showed that I burned 1734 Kcal's. Is the only factor in determining calories burned average heart rate over time? I am sure there is some algorithm that the watch uses to calculate the calories burned, but does muscle glycogen depletion come into play for calories burned?
Meaning if I worked out for 1 hour and felt near the bottom of the tank at an average heart rate of 140 and the monitor showed 900 Kcal's versus working out for 1 hour at an average heart rate of 140 but not feeling depleted did the first example actually burn more calories?
I hope this made some sort of sense, as is appears I am rambling again.
What we think, or what we know, or what we believe, is in the end, of little consequence. The only thing of consequence is what we do. -John Ruskin