I always find that I hurt myself when I do deadlifts, I also have a bad back so that doesn't help and is why I avoid them unless I'm feeling really confident my back won't cause any problems ha . I find with squats it helps build up my lower back, and butt offering me a better sense of balance. I also get simply more power out of my legs, when I cycle and stuff, and I"m sure the affect would be the same for you as a runner. I'd recommend trying it, just start off light, and slowly build yourself up. As far as body mass goes, I can't really say I've gained much body mass but that's not my goal though.