I've always been sort of antisocial; I function perfectly fine in social settings, in fact my wife thinks I'm way better at it than she is... I just don't need it the way some people seem to, in fact, I can take it or leave it. We've got some aspergers type stuff in our family, I certainly showed some traits as a kid and I may still have some holdovers from it as an adult.
I did an awful lot of boozy party-type socializing as a young adult so I also feel like I 'got some of it out of my system'... and I'm blessed with a raucous, loud family so I'm rarely 'lonely'.
But I've noticed that folks of our age- 40ish- seem to cocoon more, keep to themselves. And yeah, when people are married, with kids, etc. it's even more so. We've got some friends we've made around here who make an extra-constant effort to do things and go out and be social. I'm fine with that, but again, I like it in small doses.
I'd suggest what MN said- join a club, similar interests, find out what sorts of community stuff there is in your area, volunteer, work at a library... as far as sports go, I look around and I see a lot of atheltically, um, challenged guys my own age. So I would think not having a history or any special aptitude would be an impediment IF that sort of thing is your speed. Or, join a bowling league, something like that.