[DrScump]

"Well you are burning quite a few more carbs than fat even at a fast 1/2 or full marathon"

No, you aren't. Your liver can hold only so much glycogen, and you'll use that up well before the 10-mile mark.

[zimpenfish]

http://www.ncbi.nlm.nih.gov/books/NBK22417/ suggests that an elite athlete (they're talking about 2 hour marathons) could fuel about 1:20 of that with glycogen (~18 miles) but that they burn about equal glycogen/fat (which would give 13 miles of glycogen.)

I suspect your 10 mile mark is probably correct for anyone below "almost-elite".