The Washington Area Bicyclists Association has a good set of links to maps of local trails.
Hiking Upward has hike descriptions for several parks in Virginia and West Virginia. As suggested by the name, most are well outside the beltway, in the mountains. But a few closer-in hikes along the Potomac are included. There is a helpful summary page that allows you to pick hikes by location, or by any of the other factors they're rated by: difficulty, streams, views, solitude, and camping. On each trail's page, there's a trail map, a trail description, and a Google map that can be used to get directions. Many hikers have also posted their own reviews in comments.
If you want to plan your own route, you can use Map My Walk or Map My Run (basically the same site) to estimate mileage. Unlike Google Maps (which we also use extensively), these sites will allow you to draw a route that doesn't follow a street. In some parks, the trails are clearly identified on the maps; in others, you'll have to estimate the route based on another map.
To figure out your own car-free routes, try a combination of Google Maps and the WMATA Trip Planner. Find the park you're trying to visit, then zoom in on the Google map until you can see a nearby street address to plug into the trip planner.
Did we miss any good resources? Let us know in comments...
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