yeah, the best nutrition plan for you is a "healthy" one that you can maintain in the long-term. whether you log every calorie and macronutrient and hold to the 40-30-30 or eat a more traditional endurance diet with more carbs, or just eat "clean", its a matter of finding what foods make you feel good, and finding eating habits that you can keep up. in general i would suggest setting goals that span a week or a month at first that target something in your diet you dont like. one of my weaknesses was soda, so i decided to go a month without drinking sodas. if you find yourself eating too much fast food, set a goal to only eat it once a week, or once a month or whatever works. id proceed like this until you find yourself eating in a mostly clean manner (not alot of processed foods, esp sugars and fats, not alot of fried foods, a good mix of protein, fats and carbs with as much fresh and whole food as possible) then i would look more closely at what you are eating on a day to day basis and relate to how you feel that day, how your workouts/races/whatevers go, and then further modify what you eat based on patterns you find. i know it sounds like i takes a long time (and it will) but finding out what works for your tastes, fitness, and performance goals will be much more rewarding than trying to fit yourself into a rigid-theory diet.
and as far as comparing humans to racehorses, i feel like its a correlation that cannot be made, considering that our digestive systems are different and process different foods in different ways with different efficiencies, and that humans have more than the sole purpose of running fast for a year or two of their lives.