I need help with friends!!! Please answer?

I've always had low blood pressure and low electrolyte count, which prevented me from playing sports in high school. Don't get me wrong, I'm pretty active. I work out and jog, but I can't handle more than half an hour of intense activity. It was really bad in ninth grade, when I fainted a bunch of times. When I started college, however, I started hanging out with a new group of friends, and they're really active so a lot of the time I can't keep up with them. I don't want to tell them the reason, but I might come off sounding really weak. What should I do?