I took my first spinning class last week and had the same fear- that I would be left in the dust.
Let the instructor help you adjust the bike seat and handle bars to the right height (ask her/him if they don't offer). Pretty much the instructor encourages you to work harder, but it is pretty much at your own pace. If it gets too much you are in charge of your resistance and you can turn it down.
Because I didn't really know what I was doing, I didn't work as hard as I could have in my spin classs (I was afraid of wearing myself out early), but no one said anything to me that indicated they thought I wasn't working hard enough. I'll prolly kick it up a notch next time. Take it easy your first time and figure out what your pace is.
I say "Go for it!"
