When I was in my teens, I would often get short of breath and fatigued whenever I ran laps in gym class. I could handle running short sprints, and I had enough energy to play the games in class, but long-distance running really took it out of me. The coach just thought I was completely out of shape.
Years later, I figured out what was causing my shortness of breath and fatigue. I had an allergy to freshly cut grass, and we would have to run our laps around a grassy field. This would trigger a case of "exercise-induced asthma", a breathing condition that only happens during aerobic exercises like distance running. I was fine as long as I ran laps indoors and paced myself, but the allergens outside would trigger an asthma attack.