If you're near Tilden you want to head north on Beach Drive...out to the end at Garrett Park Road and back gives you a 30 mile loop (heavily used by cyclists so drivers kind of expect to see us).

There are routes off Beach to get out to Potomac and beyond (Tuckerman Lane features in a lot of them). I'm not as familiar with those as I do most of my longer rides down on the Northern Neck of Virginia, where I have a weekend place.

If you have a car there are lots of good routes in western Montgomery County and points west.