A couple years ago my wife asked for a divorce (we are still married but separated). It was very much a wake up call that I was very dependent upon her emotionally and I had neglected my other friendships. I did two things. I invested myself in some old friends from high school and college, but as none of them are living in my city, I realized that I really needed to invest in new friends.
Several friends that I did have were runners, and they suggested that I join one or two local running clubs which I did. I wasn't and am still not a "runner", but now I have so many friends I don't know what to do with myself. I am I'm as happy as I've ever been in terms of having friends with whom to socialize. There are presumably clubs like that in every city. Obviously would help to be a runner, but like I said, I am not. I run for social reasons not because I enjoy it.
Another interesting thing with socializing in middle-age is that 4/5 of my friends now are women. I think that's because women make better friends. They are more outgoing. They are more emotionally aware. They're more willing to invest in friendships. I think guy friendships are harder because of social expectations of of what a male friendship is - especially if like me you don't enjoy drinking or spectator sports.