This article is from the Misc Bicycles FAQs, by various authors.
There are quite a few organized centuries out there. I receive a
packet for a charity ride almost weekly now, and could never hope to
raise enough money to ride in all of them. Many recreational clubs
schedule two big centuries a year, one in the spring and one in the
fall. These rides, typically will be arrowed, sagged, and will provide
some souvenir like a T-shirt or a patch. Many people look for a
flatter ride for their first century, and I certainly wouldn't
recommend a mountainous one for a riders FIRST.
To find out about these rides, look for brochures in local bike shops,
listings in local newspapers or sports papers, listings in national
magazines like Bicycling and VeloNews, or through local clubs and of
course announcements on computer networks!