Regular old grass hay, soaked beet pulp and a high fat low protien low carb feed.
Fat burns at a lower temperature than sugar. All about heat management in endurance horses.
Also people loose sight of the fact that horses arnt like us, and they arnt like dogs. They are not carnivores. They DONOT need tons of protien. People think protein equals building muscles Soooo the more the merrier. So feed makers respond with what sells. Est food in the world wont do a manufacturer any good if it is left on the shelf. People like to see high protien numbers so that's what they put in the bag. All you are doing is creating rich, high amonia pee. While not huge deal on a casual horse. Makes a big difference in endurance. Extra unneeded protein requires large amnounts of water to process and flush. Endurance is all about hydration and heat. So feeding high protein can make it harder to keep horse hydrated. Feed I feed is 11/6 fat to protein. I supplement BCAA, a pasture multivitamin, and electrolytes, depending on weather and rides. But I'm just some smukatelly bob on the internet with a few completions and several pulls, I know alot about what NOT to do. But I do alot of reading and research. Go to the source Check out the research by Dr Garlinghouse. Pretty much considered the guru of all things food and endurance related.
Particularly good is beating the metabolic pull, Susan E. Garlinghouse, DVM horse management articles and lectures Susan E. Garlinghouse, DVM horse management articles and lectures