I was able to walk 3000 miles on a diet of rancid vegetable trans fats, flour and sugar. I did have trouble keeping the weight off during the hiking, however. The weight crept back on me slowly but surely the longer I was out there.
It's a lie that eating healthy will result in better performance. I just was hiking with a guy who has hiked 10,000 miles in the last two years on poptarts and peanut butter and candy. He was pretty young, but even in my mid-40s I was able to do almost the same (same diet, fewer miles.) There was a younger woman with us in her 20s on the poptart and peanut butter diet. She could do the miles but she was not skinny. There was an older man with us in his 60s. He was not eating any sugar or flour. He (and his wife at home) had achieved amazing results cutting only those two things from their diets. They have had all the same health-related results that are trumpeted here. Plus they lost weight without calorie-counting. The wife lost 130lbs.
What eating healthy does to you is make you have better mental health, clears up issues like snoring and sleep apnea, clears up a lot of allergies, acne and IBS things, dramatically reduces hunger for some people (my friend talked about eating three heaping plates of pasta for dinner and feeling hungrier after every plate, then falling into a coma on the couch). We compared notes and everybody we know at our age (middle-age and above) has had amazing health effects from cutting flour and sugar, health effects we could not achieve with diet and exercise alone. Cutting the flour and sugar really seems to be the common theme among us even though among us were folks who continued to eat all grains so long as they were not flour as well as not worry too much about stuff like white potatoes, quality of meats or oils, amount of sugar from fruit/dried fruit. Some of us did, some of us didn't, but we all had the same results anyway.
Female, 5'3", 50, Max squat: 202.5lbs. Max deadlift: 225 x 3.