If you're running a lot, you're gonna be burning a fair amount of fuel.
Yes, and no. One of the reasons that runners try to drop weight is because carrying it around for 20+ miles makes the race much more grueling.
The less you weigh the less you burn, just think about carying a 5 lb weight 20+ miles! If you don't need it to run it's gotta go!
When I started running and was 125 lbs I burned about 100 calories running a mile-- now I think I burn about 65. That's only 1690 calories in a marathon! Not even a lb of fat. (3500 cal)