My friend Tom was with me past year in a quick visit to the Col of Tourmalet from Bilbao. We left Bilbao at 7:00 and came back at 21:00. During the day, including a bike ride climbing the Tourmalet, Tom only ate a coffe, a croissan and a cereals bar. But he noticed I ate a brunch in the Restaurant of the summit of the Tourmalet, with fried eggs, jam, chips, coffe, etc.
So Tom is convinced that I won't be able to finish the Marathon without going into a Restaurant and eat a good meal.
And, I don't know, perhaps he's right.