I had a lot of trouble breathing when I first started doing hills and then finally learned that I had exercise-induced asthma. I've probably always had it but I never exerted myself that much before. With my inhaler, I have no problems doing the hills anymore (well, I still suck at them but at least I can breathe). I also wear a HR monitor and I know that if get too close to my max (185), I have trouble breathing so I slow down a bit. I am hoping, like the other posters said, that once I get in better shape, it won't be as much of a problem...