To be fair, regarding ROTC, there isn't a need to physically prepare for it if you are coming from a high school athletic background. Compared to varsity sports, it's a cake walk. The real challenge is academic, mental and time management.
Loading up on courses in the Fall/Spring because Summers are going to be dedicated to military training which doesn't leave a lot of room for Summer school. Which means you will be taking 5 courses a semester, plus military science, leadership labs and morning PT.
Time management is the biggest challenge, the physical stuff is an after thought. A typical day would be physical training at 6 AM, classes in the morning/noon, lab work in the afternoon (land nav training or some shit) which leaves your evenings for studying.
Sitting down and studying calculus II followed by some exciting reading about Spanish exploration in the Americas after a long ass day while you are being tempted to go party with friends or Netflix and chill with a girl is hard.