I have Becker MC, and if I don't exercise regularly I feel the effects of the MC much more. I have had the MC effect just about every muscle.
MC aside, I had a friend who, in high school, was as thin as a rail and would pretty much eat as much as anything he wanted. I used to warn him that it was going to catch up with him someday. Surely enough, by the time he hit 30 he was probably somewhere around 30lbs overweight. There are also things like heart/cardiovascular disease to consider.
You might want to get some books on "eating clean" and give it a try for a month and see if you see any improvement in the way you feel, even if you don't feel bad, you might not feel as good as you could.