Add me to the "not built to run" camp. I'm a "thick" person...stocky build, plenty of muscle with a nice layer of fat over them. I have asthma, but it rarely flares up during exercise. My biggest limiter with running, is my lower back. I have a vertebral slippage at my L5/S1...have had it most of my life. However, it didn't become symptomatic until my late-30's...which coincidentally, is around the time when I started to run and also do heavy weightlifting. There isn't a doubt in my mind, that the tremendous load of the heavy squatting and bone-jarring impact of running, is what got me to this point. For what it's worth, I no longer do loaded squats (only with a ball against the wall)...but I DO still run, if you can call it that. I run on the treadmill at the gym, 3x a week, slowly (5.5 - 6.0mph) for 30-40 minutes max. In all honesty, I don't really enjoy it much at all...because it hurts a fair amount. But I do it, because it's the only cardio activity that helps me keep my weight stable. Yeah, it's probably not the smartest thing for me to do...and I argue that point with myself on a daily basis. I'm sure my running days are "numbered", but until then...I'll keep slogging along and do what I can until I can't tolerate the pain any longer.