Centuries are worth it. I do unsupported centuries. This year, I plan to do one each month I'm off (I teach). I average about 12 to 13 miles per hour, and bring a change of clothes.
I found that Hammer Nutrition products perpetuem, recoverite, help me get back on the bike and keep me going. I usually take the next day off.
BTW, I'm 59 years old. I see things on centuries that take my breath away, wetlands, full moons over lakes, spider webs forming on my bike as I ride. Plus, the ability to ride these distances is a personal affront to my arthritis.