Washington Area Bicyclist Association (WABA) lists places to ride, people to ride with, clubs, gear and community events for every level of cyclist.
RowingLinks.com lists a wide range of local and national rowing programs.
DC Metro-area and national running clubs abound; find one that meets your personal fitness goals.
Planet Swim highlights many aquatic sports clubs including swimming, water polo, diving, and synchronized swimming.
Walking is one of the most popular activities for maintaining your physical fitness. American Volkssport Association lists local walking clubs, guided walking tours, and an events calendar.