Well when I started using the bikes I wasn't working out very hard but I think its best to start small and build up.
See if you can stand on the bike while working out, just start with intervals of standing up and build up.
Turn up that resistance, keep on varying it, for most of the time I try to get a steep resistance where I can still go fast then have little intervals of low resistance going fast sitting down and then intervals with a really high resistance standing up but I go slower so my whole body is really pushing myself through.
Also try getting your upper body down lower and almost sit but just keep your bum hovering over the seat, just hits the muscles at a different angle.
Take rests, maybe even have a few weights beside the bike so you can get off do some lifts, give your legs a break anyway and then you can jump back on.
Make sure you don't fall of the bike.
a girl at my circuit class did, she didn't get her foot out of the strap before getting off and your legs can be quite shaky so take it easy.
Also my foot nearly slipped off one of the bikes without the straps and you don't want to bang your self off the bar ouch.