Practically a newbie
I consider myself in the same boat, because, though I was on a gymnastics team in high school, I wasn't... "good". Now eight years (and about 25 pounds) have gone by and I'm pretty sure I'll be starting back at square one.
A little about myself: I did high school gymnastics, which (at least in my division) was a different animal from USA gymnastics. My team didn't have tryouts and though we competed level 9 rules, anybody was allowed to compete. Had I been in club, I doubt I would have cleared level 4. This resulted in some colorful scores (my average bars score was in the 3s) Some people would think that's embarrassing, I personally think it was awesome to have the chance. (Can you tell I got a lot of "good sport" awards?) I got to compete against some amazing athletes, and the team environment was a lot of fun (I'm so jealous of NCAA teams!)
Anyways, back to present day. I recently took a part-time job as a secretary for a gym, and thus have the whole gym to myself when I feel like staying late. Pretty much my little-gymnast-version-of-myself's dream. I won't have much in the way of coaching, unless I can bribe one of my coworkers to stick around, but I'm still just plain ecstatic about getting in there again, crashing around on the tumbl-trak, and getting up on the beam again. (My favorite event.. which, I'll brag that I beat a level 10 teammate once...)
My advice is to make up routines for yourself. Pick one you like off a video, and make yourself a "dream routine" replacing the things you can't do with simple moves. So obviously, you'll start with almost nothing in it (poses, jumps, etc) but you can replace skills as you get more comfortable on the equipment. (ie, replacing a leap pass with a somersault or cartwheel) I think it really helps keep it interesting if you have a concrete routine to work on. You'll get bored quickly if you're just doing one skill repetitively.
Another thing to consider is a private tumbling lesson. Less specific to gymnastics, I've seen many in the 18+ crowd take those lessons for martial arts or dance, so you might want to check with those types of studios to see if they have something. Those coaches might be more accustomed to working with and spotting an older (and, ahem, bigger) person.
Hope these things help, because really, unless you happen to find some fellow adult gymmies, it'll be a solo journey. But still a fun one! Good luck!