Does your gym do different intensity spin classes?
It is hard, but I just do what I can - my fastest speed isn't the same as someone else's, everyone works to different levels and so will you. As long as you vaguely follow what the instructor is telling you, you will be fine!!
My one tip, which a friend gave me before my first class, is that when they tell you to turn the resistance up, don't do it as much, as it makes it so hard!! I usually pretend
and I've been going to spin for 8 months now!!x