Shenandoah is the closest national park to Washington, DC. You can drive to the northern end in just over an hour. It is also a very long park. It contains the northern part of the Blue Ridge Mountains (which stretch down into North Carolina), and they are a long and narrow range. So the park is almost 100 miles long, and rarely more than a few miles wide, so some parts are quite a bit further away. It also is good for backpacking. The Appalachian Trail goes for the entire length, and there are some other places, particularly where the park is a bit wider, that you can do some short loops. When I got back into backpacking in 2005, this was where I went. This trip is in the fall, when there was some color to the trees, something this park is famous for.