I've found it difficult to stay on a real low carb diet at 11 hours exercise. At about 6 hours, I began to have headaches. Active.com, a good resource, says that endurance athletes should get about 15% from protein, 60-70% from carbs, and the rest fats. Here's a link to a recent article:

I did strict Atkins for five months, then started triathlon training seriously, and had to give it up. I have maintained my weight (from Atkins loss) for almost a year at this level.

I will be doing the MS-150 in my state this weekend (cycling 150 miles in 2 days).

I think low carb is a super way to go to get the weight off. However, at higher levels of exercise, and especially long duration exercise, it is crucial for your body to have fuel to burn.