5'11" and 120 isn't that bad. When I was in my early 20's I did a lot of hard core rock climbing, even some competitive, and was 5'11" and 128lbs. I was tested at 4% body fat. I could do 60 pull ups though and was pretty ripped. Not huge muscles but a lot of definition and strength for my size.

Now I'm 185lbs. That's what 24 years of smoking weed and having the muchies will do to you.