I believe that the marathon is about equal amounts of physical and mental toughness. I think the mental toughness aspect of the race is often times more important than the physical side. There comes a time in every race, and in every training run, when most runners want to quit. I can say this unequivocally, there comes a time, usually multiple times, in every run when I want to quit.
Running is hard. Very hard. I have to continually and constantly resist the urge to quit.