When a hobby is taken too seriously, it can then become a chore.

I've only been back on a bike after a gap of over 30 years for just over a week, so I'm no spring chicken. I'm enjoying cycling and have thought about joining a club. What puts me off the idea is exactly what's happened to you.

Go back to what you enjoyed when you first got your bike.

You've actually answered your own question in your first paragraph:

Up until then, almost all my cycling was alone - long distance commuting or just plain fun trips and just cycling everywhere.

the operative word being...FUN.

If you want to ride out with other people, why not find a couple of cycling pals to go out with - at your own pace - and enjoy. That's what I intend doing as I don't think I'll ever be fast (or fit) enough to join a cycling club. I'd rather just meander along at my own pace and enjoy my bike.