I like weekend long runs at these places (so why not promote?):
Nike Run Club. Saturdays 8 am Georgetown Nike, M Street, 4 or 6 or 8 mile packs, paces 645 and up, sociable. Mostly 20 somethings, but not just. Free.
Pacers Clarendon: Saturdays 8am 3100 Clarendon, newish for 2015. The weekday morning crew is fast kids, and evenings more relaxed. We'll see what mix develops on saturdays. Free.
DC Road Runners: Saturdays 8am Iwo Jima Monument (actually north of it across the street since park service acted pushy). Distance 10 miles and up, since they train for marathons. But a decent number turn back early. Boomers to Gen Y. Annual membership isn't much.