I struggle with this as well.
I wonder if maintaining a social life could be more difficult today, because the "social infrastructure" isn't what it used to be. Your society's habits don't carry you as much as your parents', so you have to do more work yourself.
And then you may even not know how to do the work - as other commenter here said: being social is a skill that is learned. I haven't figured it out the puzzle myself. I'm a few years after college where many people just moved away and I'm in the same spiral that other people in my situation describe.
My coping approach has been more like "leave the house" rather than "meet new people" which sorta works for me, for now. Being around people is also humanizing. At the risk of describing something banal, I'll describe my learnings below.
One of the first struggles I've come across is that staying up to date with what's happening in the city is work. As I described in the beginning, the reason that building social capital for yourself is so hard is that the social infrastructure isn't there. One of the best ways to learn what's going on in the city is to have something mentioned to you in a conversation, or to be invited by someone who's already going. You passively learn or participate by leaning back on the other person. It's expensive to be poor.
The way reduce the amount of work is to find cyclical events. That way, you learn about the date just once then keep coming. I've found that the best way to learn about them is to subscribe to e-mail newsletters of cultural institutions (museums, galleries, operas, theaters, cultural centers). Some will never send you an e-mail, but some are pretty active. Sometimes the e-mails contain info that isn't available anywhere else - my local museum holds free visits with a tour guide every Sunday at 9AM, but that isn't mentioned anywhere. The benefit with e-mail is that you're passively being poked by the institution about an event. What doesn't work in such a way is e.g. Instagram, where you have to open the app and doomscroll through unrelated things in the eventual hope of finding some event.
Instead of e-mail you can also sometimes use RSS but completing the list of institutions, finding the feeds and then remembering to read through them is very work-like, as opposed to e-mail.
This of course doesn't solve the problem of loneliness, as you'll be going somewhere, but still alone. In spoonfeeding-a-social-novice terms, random events are a bit further in the social pipeline, where you're "not supposed to" go alone, but where you're going because you've already met people people to go with in earlier in the social pipeline in hobby groups.
I used to have a barefoot pair for the gym. I liked that they are much lower profile than sneakers, thus were more compact in my gym bag.