I wish more clubs would offer longer spin classes. I'm usually just getting warmed up after an hour. My old club used to have back to back classes so I would stay and do both. Now I usually (if the class is open or not being used) go in 30 minutes before to warm up and then stay another 30 - 60 minutes after class. Got some good tunes on the iPod and I'm good to go!

I do think spin is tough, at least a good class is. But it is different than riding where you also have to contend with gravity and road feel, friction and wind. Both are good cross-training tools for each other. After an injury I was unable to ride on the road but was able to do spin and whent he fractured healed I was able to do a century without much loss in fitness.