Ayres identifies the problem of refueling as the biggest difference between marathons and ultras, and the hardest thing he had to figure out in his transition to longer lengths. Glycogen stored in your muscles will provide adequate energy to go 18 to 20 miles, so well-trained runners can almost cover a marathon distance without much thought to refueling. But ultrarunners necessarily need to carry some sort of fuel along for the ride.
In 'The Longest Race,' Ayres tells a story of reaching exhaustion at the 70-mile point of the Angeles Crest 100 Mile and spending half an hour at the aid station at the foot of a steep mountain trying to figure out what he could eat without throwing up (his answer: a boiled potato). Crucially, Ayers points out that food is not a race-day or night-before proposition but a months-long – ideally years-long – affair.
Ayres has been on a Paleo-type, vegetarian diet since 1954, way before it even had a name (for reasons of environmentalism and personal preference, he has since cut out Paleo's heavy reliance on meat). "A lot of people might consider me some kind of nutritional fanatic. But how I look at it, the way I eat is in line with the way humans ate before the Industrial Revolution." For him, that means no white flour, refined sugar, or processed foods. Sometimes, he says, we have to "override our evolution" if we want to survive as a civilization.