I have no doubt, that you can run marathons on a low carb diet. I've been on a low carb diet for 1 1/2 year and I don't think, that my endurance has decreased.

There's also a very good scientific paper about ketogenic diets and physical performance:
http://www.nutritionandmetabolism.com/content/1/1/2


The problem is, most people won't believe in the superiority of low carb diets, as long as all professional athlets follow a high carb diet.

It's really strange, that nobody can tell just a single name of a professional athlet following a low carb diet. As far as I know, Lance Armstrong was not on a low carb diet, he was probably just better doped than the others :-)