I agree about PUGs and would definitely recommend being in a guild, but don’t let experience with just one guild put you off - there are hundreds out there of all sizes and you will be able to find one which suits you. There are also some great communities built around players in your situation with literally thousands of members. There’s a discord somewhere where they organise runs, I’ll try and track them down and post here.
Though I’m in a great guild now I played for a long time solo after my guild went awol during Mists. I just relaxed and did my own thing - I’d say don’t feel under pressure to do anything in particular, there’s so much to do in the game.
Sorry this is on r*ddit but it’s such a useful list I couldn’t not post it: https://www.reddit.com/r/wow/wiki/discord/