There’s always the Washington-Old Dominion trail that goes from Arlington to Purcellville (around 40 miles long). Its a well maintained trail that’s just for bikers/runner/walkers. Paved the whole way. You could either drive there and start from the trail head, or there are ways to navigate there from DC to the trail. But if you’re uncomfortable around traffic, being able to drive to the start is probably your best bet. Hope that helps!