Stay Comfortable. The biggest complaint I hear on centuries is from riders with sore butts. The first step in eliminating a sore butt and maintaining your comfort on a century is to get a bike that is properly fitted to you.
While I know cyclists that take their favorite painkillers before, during and or after a century, I never do. Painkillers don’t address the root cause of the problem. I find that there are many steps that you can take to eliminate the cause of a sore butt and I’ve discussed them in detail in my blog Sore Butt.
However, perhaps the best recommendation to eliminate a sore butt on a century is to change your riding position often. By doing so, you are are reducing the pressure on the same points on your butt over the six or seven hours in the saddle.