Take. Things. Slow.

You'll burn out reaaaaaally quick if you attempt to run everything at a quick pace just to catch up.

You've got a CRAP TON of quests and sidequests to catch up on, and leveling has DEFINITELY gotten easier since the old days. You'll find yourself out leveling far quicker than the story itself. But don't let that bother you. Play different classes to spread out the exp gain if needed.

See if you can find an active FC to run things with and get some advice from. Don't just sit around on your own; having some backup will make things easier to run especially if the idea of running content with random is intimidating.

But above all else. Have fun. Plain and simple. And don't sweat the small stuff. Welcome back and enjoy.