I believe there are two or three sources for long distance driving pain:
1. If the seat doesn't breath, then the moisture build up can cause irritation. (This is what I was getting at with the leather.)
2. If you don't move your position regularly, you can get some pain from holding a position for too long. (This is easy to do if you are concentrating on driving in moderate to heavy traffic.)
3. Some people are more sensitive to seating positions than others. (lumbar support, seat length compared to leg length, etc.)