If you are going to get serious about riding a bike for long distances, you really have to wear padded riding shorts.
These are special made shorts that have two pieces of padding in the butt. As any rider will tell you, even if you have the greatest seat in the world your butt is going to get sore on a long ride. Adding a little extra padding goes a long way.
You can find these shorts in most sports and bikes stores. They are relatively inexpensive and they also offer all the normal advantages of bike shorts. I think they are really a must have for serious riders.