Rock Creek Park is the main stretch of woodland that runs through the city's northwest corridor. If you want to escape the mob of tourists on the Mall or you just want to spend a bit of time in an urban oasis, head to Rock Creek Park. Any section between the Kennedy Center and Pierce Mill has features aplenty—running/biking trails that follow the twists and turns of the creek, tennis courts, idyllic scenery, and the occasional distraction like the National Zoo. On weekends, Beach Drive, located just north of Pierce Mill, is closed to vehicular traffic, turning it into an ideal running/biking trail. To enter the park at Pierce Mill, take the Metro to Van Ness/UDC and head down Tilden St. It will be about a 15-minute walk to the mill.
For those looking for a more strenuous workout, a hike or run on the Western Ridge Trail might just fit the bill. You can get more information about the trail at http://www.rockcreekrunner.com/2011/02/03/trail-running-in-dc-western-ridge-trail/