WHat does the body not knowing whether you are running 20k or 42k have to with it?
Of course the numbers given are not constant throughout the race if that's what you are thinking.
Canova's data represents a way to express the TOTAL energy requirement and then breaks that down into the TOTAL carb and fat requirement.
Maybe I misrepresented something but some more data to back the concept up:
"Sjodin and Svedenhag (1995) observed that among elite marathon runners they studied, those with the most economic run spent .816 kcal per km and per kilogram of body weight, those with a higher running cost spent .988 kcal. Energy expenditure was on average .908..."
So for a 70kg man=
.908 kcal/kg/km X 70kg x 42.2km=2682 kcal
Using data by O'brien it can be seen that about 1500kcal comes from muscle glycogen and 400kcal from the liver glycogen.
That equals roughly 1900kcal....So how do we get that other 800kcal needed????????
Use your head.
If you want to look at it another way, RER is another way to quantify the ratio of fat to carb used. Top notch marathoners all run at an RER below 1.00 (generally in the .9x range for most). Meaning that fat is used even in the early stages.