Slow Burn by Stu Mittleman is all about low-carb although the term doesn't appear in the book. His take is by controlling heart rate, you make the body burn fat. He goes for distance over speed. The back page mentions his 3000 mile run from San Diego to NYC. I'm guessing that with time the body get better at using fat or energy. But fat burns in a carb flame. The trick is finding the least amount of carb your body needs. I've done 50 mile races on very low carb. I ran slowly but I was running.