Speaking of Greg McMillan (see previous post), he was one of the first to speculate that long training runs without taking in calories may enhance fat burning and race performance.
..a great way to ensure that you will deplete your carbohydrate stores on these long, steady runs is to not eat any carbohydrates immediately before or during the run. Any carbohydrates ingested will be used by the body for fuel, and we don't want this. 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 Marathon Long Run
Now there evidence to support Gregs advice- the Train Low, Race High theory.
"Training with lower levels of glycogen in the muscles appears to elicit greater endurance adaptations in muscles, such as improved aerobic efficiency and increased capacity to burn fat compared to training with high levels of muscle glycogen" -Peak Performance Online
(McMillan recommends working up to 2.5 to 3.5 hour long training runs with little or no carbohydrate intake)