Everybody knows that during long runs, the farther you go, and the more carbohydrates you deplete, the more your leg muscles rely on fat as a primary source of fuel. Done on a consistent basis, your muscle cells adapt by becoming more efficient at burning fat.
For this reason, marathon coach Greg McMillan suggests long steady runs up to 2.5 or 3 hours WITHOUT pre loading or taking in carbohydrates! "We want to deny the body carbohydrates in these runs so that the muscles will become better at sparing the carbohydrate stores, more efficient at burning fat and used to running with lowered blood glucose levels"
The potential problem with this advice is obvious- a lower level of performance. That's why McMillian also recommends a Fast Finish progressive type long run to simulate your actual marathon, where pre loading and regular intake of carbohydrates is encouraged.
For more info, go to mcmillanrunning.com
MY COMMENT: This may or may not work for you. I just recall my early days of high mileage marathon training in the 70s- when we never took carbohydrates during long runs (or even in races for that matter until Gatorade became popular!)