Interesting, you may want to check out Phil Maffetone and his 180 formula to calculate your heartrate to train at to stay in fat burning mode. I have his Big Book of Endurance Training and am almost done. According to his theory, if your low to moderate carb you want to stay in fat burning mode as you have upwards of 40K of calories you can tap into from your fat storage compared to something along the lines of 2-2,200K of calories of glycogen stored in your muscles to access. Go from sugar burner to fat burner, it will take a few weeks. That way you may have something left in the tank to really put the push on at the finish and avoid overtraining. At least that's his theory, we'll see as I plan on trying out his ideas with my mountainbiking.
You'll never see the light if you're in someone else's shadow, or said another way, life is like a dog sled team, if you're not the lead dog, the scenery never changes