.... I think it just comes down to the dude.
In post-boot camp MCT, in 98-degree tempts, we each humped about 90-lbs 23.5 miles up a mountain range; down that range; to the beach; along the beach; and up another mountain range in 5.5 hours. It easily was the hardest thing I ever did physically. After 4 months of continuous, mostly-vice-free physical training, 30% of our unit couldn't do it and dropped out. I barely made it. 6'-5" 250-lb Private Brown, who was all muscle, wasn't really even winded at any point carrying the same load.
I think one variable is that some people's lungs are more efficient at processing oxygen than others. IIRC, aside from the doping, Lance Armstrong's lungs could process something like 30% more oxygen in a certain amount of time than an average male of his same size. Just good genetics.
