Have a DP now but was in this situation for a long time.
You have a dog, so that's a good start! Lots of walks and yes maybe meet ups. Even if the people are older, it can be great having older friends. I went away to dog friendly places a couple of times, just for a night or two. Scary at first but loved it.
Coupled up friends would still want to do stuff, it just meant you had to arrange stuff earlier, particularly if there were kids. Or I'd visit my family locally, or go on weekends away to friends or family who live further afield - even the ones in couples as I could get on with the partners too.
Gym, swimming, yoga. PJ days on the sofa watching whatever the hell I fancied. Reading and fannying about online.
Meetup I used a few times, more when I lived abroad but also a few times in UK, it was good, met some nice people. I took an evening class in philosophy because it was something I wanted to learn more about, that was in the week actually but it did lead to meeting friends I could make plans with at the weekend. I'd go walk round the local market or town or venture out to nearby places and have a wander.
Still do all/most of these things now because they were things I discovered I really liked and enjoyed, maybe not as often but as often as I can, because it's important to me to keep my own space and independence and because all that single time helped me learn that these were the things that make me tick so I won't give them up for someone else again.