It took me a couple weeks to get used to it and after that have not had any trouble whatsoever. I'm a 3-4 days a week spinner myself. Those first couple of weeks every muscle in my body was sore including my arms!
I had to strengthen my core to sit on the thing correctly. I remember that sitting up without holding on was hard. It's quite comfortable to do so now.
Keep it up! I love spinning!
And I know you said made it 30 minutes because of the sore butt thing, but just in case the resistance was too hard keep in mind that you really can take it at your own pace. The resistance and speed is up to you regardless of what the instructor says. Mimic what the instructor does but at your own pace. No one will mind or probably even notice.