JST Nutrition, which designed the supplements bundle for the Steve Prescott Engage Mutual End-to-End Extreme challenge, has these tips:
Include plenty of low fat, high carbohydrate foods for energy, and fluids for hydration. Carbohydrates are the body's preferred source of energy for cyclists. Since you are constantly burning carbs to fuel the journey, you must regularly replace them with a high carbohydrate diet. Muscle mass, energy bars and fluids, and L Glutamine all help refuel riders at the end of a day.
As the cyclists will be riding for long periods they'll need to replace carbs regularly. It's recommended about 30 to 40 grams of carbohydrates every 30 minutes they ride beyond their initial 60 minutes. It's important to replenish the body and prepare for their next ride.
Make a conscious effort to drink fluids before, during and after riding to stay hydrated. Becoming dehydrated is a real danger so it is important to take on lots of fluids - at least 1 pint of fluid immediately before a ride and another pint every half hour during a ride, not forgetting the importance of taking on fluids to replenish those lost fluids after a ride.