Have you checked into any local bicycle clubs in your area? I'm affiliated with a bike club in the Chicago area and we do a double century around the third week of June. Another alternative is to ride a Brevet. The Great Lakes Randonneurs has a 300K-186mi scheduled on May 15th. The ride takes place in Delavan, WI. Here are two links you might find of interest.
http://www.rusa.org
http://glrand.home.mchsi.com/
Hope this information helps.
Tom