Why do most professional endurance athletes practice agnostic healthy eating instead of following one of the many diets with names? The primary reason is that elite athletes are willing to do whatever it takes with their diet to maximize their performance, but they don't want to do more than what's necessary. Many of them would indeed maintain the average American diet of potato chips, white bread and energy drinks if they could, but they can't. Instead, they adopt the least restrictive way of eating that's sufficient to support maximum endurance performance, and that's agnostic healthy eating.
If it were necessary to avoid animal foods to maximize endurance performance, most professional endurance athletes would be vegan. They wouldn't be happy about it, but they would do it. Likewise, if it were necessary to avoid dairy, grains and legumes to win races, then the pros would go Paleo, and if it were necessary to maintain a precise macronutrient breakdown of 40 percent carbs, 30 percent fat and 30 percent protein, they would all be on the Zone Diet, but these things aren't necessary either.
There's One RuleAgnostic healthy eating has no particular rules, but there's a simple guideline that makes it easier to practice consistently.
There are 10 basic types of foods: vegetables, fruit, nuts and seeds, high-quality meats and fish, whole grains, dairy, refined grains, low-quality meats, sweets and fried foods. The first two types are classified as "mandatory," the next four as "recommended" and the last four as "acceptable."
The one rule of agnostic healthy eating is this: Each week you must eat each of the two mandatory foods more often than any other food type; you must eat any of the recommended food types you choose to eat more often an any of the acceptable food types; and you must eat any of the acceptable food types you choose to eat less often than any of the mandatory or recommended food types.
The rest is details, such as determining how to count composite foods, sauces and dressings, alcohol and coffee drinks, and so forth. These can be found in my book, The New Rules of Marathon and Half-Marathon Nutrition. There are infinite ways to practice agnostic healthy eating, one of which is certain to be perfect for you.