Have you ever been fitted to your bike at a good LBS? I know your situation is different but a lot of pain problems in somewhat new riders is poor rider setup. Having your handle bars too high or too low combined with a seat that is too high or too low and angled up/down too much will cause pain to your back side. Most decent LBS will also have a pad you can sit on that will show exactly where your sit bones are to help get the proper width seat. Also you may just need to ride more. I used to ride about 10-15 miles almost every day then I took about a 4 year break due to some life and career changes and am now dealing with slight soreness now that I'm riding again. My bike hasn't changed, I just haven't built back my riding tolerance.