Julie, spinning is awesome! It's by far my favorite cardio to do in the gym. At the gym I belong to, it's a 45 minute class. It's done in the dark (with blacklights!) and loud, makes-you-want-to-move music. You'll do a combination of hills, sprints, and flat road riding. Each instructor is different, in terms of how the ride is structured, but the great thing is that YOU'RE in charge of the resistance on your bike. If it's too easy, a turn of the knob makes it harder. If it's too hard, you can back off until you're comfortable.
It might hurt your butt for the first couple of times you try it. Some gyms have padded seats you can request; you can bring your own padded seat; you can buy padded shorts; or, you can suck it up and your butt gets used to it after 4 or 5 rides.
The other nice thing is that there are like 6 spin classes per day, so find one that works within your schedule. I, personally, like going at 6am before work, but there should be at least a couple of evening options too.
Oh, you'll also notice quite a few people with special spinning shoes, that clip directly on the pedals. You do NOT need to buy these, unless you really want to. You can just wear your running shoes that fit into the toe cages. (I held out about 6 months before buying them, and then got these really, really ugly ones in a clearance rack for $30. They can be quite expensive, and it's definitely not a necessity).
Good luck! Let me know if you like it!
I'm weighing in today at 180.