Low carb for endurance training is all about becoming fuel efficient. Endurance training in essence is all about how fast can you move over an extended distance without the body and mind giving way…
Having the body become metabolically flexible (good fat and carb burner) allows you to stave of bonking and keeps a constant supply of energy for your brain and muscles (additionally with ketones).
Becoming fat adapted has had to be one of the biggest performance enhancers for my endurance training results.
The Morris Method 