Colin Fletcher, the “father of backpacking” was a big proponent in the 1980s. He moved to California in the 1950s, so it was a natural.
His first book talking about his California walk in 1958 laid out his diet in meticulous detail. He carried a 60+ pound backpack. He ate the same thing every day, basically beef & gravy over mashed potatoes.
He also consumed about 4 pounds of pure sugar every week, on top of the massive protein intake. Kool-Aid, in Tea, “sprinkle liberally in everything”, and Raisins, Chocolate, etc.
By the 1980s he was on the anti-sugar bandwagon. But he wasn’t averaging 18 miles a day with a 60 pound backpack either. Veggies have their place (right next to the mashed potatoes) but they are not really adequate for heavy exertion.
The connection between diet and athletic performance was never intuitive for me. When I was in my 20s I took several bicycle trips down the 180 mile C&O towpath. On my first two trips, I packed raisins, carrots, pita bread, and parmesan cheese, because it would not spoil in the heat. Fit as I was, I still suffered terrible leg cramps and fatigue. Later, and even through my sixties, I made the same bike trips (camping all the way) but I learned to pack better meals, mostly freeze dried food and powdered Gatorade. there were no cramps and I had a pleasant, easy +180 mile five day ride.