I was in the same spot you were 2 years ago. I borrowed my sons Freespirit to see if I would enjoy ridding. Yes at first my butt hurt but it did get better. The thing that really helped was the pair of padded riding shorts my son gave me.
I moved up to a Specialized Roubaix this year and have no problem with 50+ mile rides. It just takes time and the discomfort gets less as you get a better fitting bike and become accustomed longer rides.