Isn't this the big problem of post-history? Ionization and Atomization. We can be so close - ie running with each other or working together but still never connect, we still repulse each other (ionization). Those we are close with, like our college friends, are usually separated from us by our own 'atomized' lives - jobs, family, responsibilities, etc...."We kiss on the mouth but still cough down our sleeves"
This idea is explored by Houllebecq and the anime Neon Genesis Evangelion. Go figure the French and Japanese would be talking about this in the 90s while Americans were concerned with Irony and the hollowness of consumerism.
Maybe the internet and forums like this can provide some 'way out' of this condition....I'm not being facetious - almost everything is virtual now. I think that the phenomenon where people met most of their friends through work (what my parents experienced) was mostly engineered, so in a way it is a very virtual environment to meet people. I've seen the argument that colleges are 'sorting mechanisms' where similar people are grouped together, this is also a virtual environment - of course one makes a lot of friends in college because chances are that some people are at whatever university you're at for very similar reasons to yourself (this doesn't make the relationships any less special though!).
The interesting thing about the internet is that you have a lot more choice over the places you go and the people you interact with than a job. This is good in some regard, because it makes it easier to find people with shared interests (read: like-minded) but also bad because it reinforces the notion (fed to us by social media and public opinion engineers) that our interests decide who we are - that they form our identity. People aren't their interests, those are what go on a college application or a in a 'profile'. It's much easier to see people as real people in the real world than it is on a place like LR. People seem fake because they see themselves as a collection of their interests, which is a very fake representation of a person, this problem will likely only get worse as everything is turned into internet 2.0
I guess the thing is that the nature of the relationship is different - on a place like letsrun you browse the forums enough to start recognizing names, reply to different posts and eventually you will develop a relationship with some posters. I remember on training threads from a few years ago being excited to hear about certain posters race results or their big workouts - maybe that's not friendship strictly speaking but it's definitely some form of relationship.
The odd thing is that a lot of these relationships are what might be called 'parasocial' - You reading my post does not mean that I am reading your post and my response to your post does not necessitate a response from you. It's a weird kind of one-sided relationship where you don't know if the other person is actually seeing, reading, or understanding what you're saying. Obviously the lack of face-to-face contact and tone of speech also makes things a bit harder to interpret. The parasociality aspect of the internet is turned up a notch on places like twitter where big accounts have many followers reading and replying to their posts.
But to answer your question - what do people in their 30s and 40s do for a social life? It's up to you to decide that! I'm sure that most other people your age are asking the same question!