Most of my century rides are done solo. I ride solo most of the time, work five week days and teach all day Saturday. So, during the spring, summer and fall I get short 15 to 25 mile rides in after work, and longer rides every Sunday. Many of the longer Sunday rides end up being century rides. Most are not planned. It's just that when I finally get home, I've ridden 100+ miles on that day. The lack of structure to these rides has been a great release from the demands of work and a therapy of sorts.