Oh you sound just like me at Easter. I had some time off, and I decided to try some classes at my gym!
1. Ask at reception for a better description of the classes. At my gym all reception and training staff have to do classes, so someone will be able to tell you what it was like, or ask in the locker room if you feel comfortable. Or sign for the class, go early and have a chat to the instructor.
2. You've been exercising a while, you'll be fine! There will be people fitter than you, unfitter than you, and at your level. Most classes stop after each song for a drink and a rest! In my gym people walk out early all the time, or just have a rest to get their breath back.
3. The whole point of doing classes is to improve. I am forever going the wrong way in Body Combat, and Hip Hop dancing, but half the class seem to be uncoordinated, and sometimes our instructors think they are the ones doing it wrong
I try and stand behind someone who looks like they know what they are doing, and copy them.
4. Prepare to be the fattest there. I am nearly always the fattest in class, but I am not the most unfit by a loooooooooonnng shot. I wer my funky addidas tank with a wee bit of lip gloss, and fake tan my upper body, and just smile my way through. No one will care that your the fattest, except you.
Go for it!!! Classes have really helped me get some variety and fun back into my workouts