Perhap... and it's just a hypothesis, that the problem with using carbohydrates as fuel is that it burns too quick. If the athlete was completely keto-adapted, theoretically, he/she should have better endurance.

I'm not a long distance runner but I do 1.5-2 hours of martial arts 4 times a week and my energy level doesn't suffer during class. In fact, most times I'm bouncing off the walls at the end. I also do weights on 2 other days with one day off.

I'm on the zero-carb diet btw.