It's the little decisions throughout the final 1/2 to 1/3 of the race that really will dictate how you feel at the end. The little decisions to go a little harder than you think you can, or to push a little faster up this hill, or try to stick with the person who just passed you. Those little decisions will dictate how you feel at the end.
I say last half or third because obviously going out like an idiot will hurt, but everyone knows how to do that. The hard decisions come in those final sections.