I take it on base. Great class; burn a lot of fat. However, it hurts your butt bone. I went to Wal-mart and bought that seat cover MargeInCharge mentioned....much more comfortable! Great investment!! if you can't afford it, just keep going and you'll eventually build up some callouses on your butt! lol
As far as your fitness level...heed this: work at YOUR own pace! Don't worry about what everybody else is doing and try to keep up...you'll hurt yourself, and be so sore the next day, that you won't be able to go back. Push yourself to get through it, and don't overdo it.
I went yesterday, and they were doing all these up and down routines, that I absolutely just could not participate in. When I have to stand up and peddle, my heart rate shoots up to 180-190. I just sit in the back and do my own thing, to my own pace....when I get stronger, I'll do more standing routines. That's what I suggest you do. If your instructor is a good one, they'll recommend the same thing...do what you can, and slowly build up to higher work load. If you get an instructor that barks at you, and is an ass...I'd find another one.