It's very easy - I do it virtually every time I fly out of DCA.

For terminal A, the "south" way is better. Just walk south along Crystal Drive, past the Chipotle and the Starbucks, on the eastern side of the street. You'll see the ramp coming from down from the airport by the volleyball courts - continue walking up the ramp, which will wind to the left, and keep walking - follow the trail basically. You cross the GW Parkway on the bridge, and once on airport grounds, there will be (tiny) signs for the terminals. The trail is more or less marked with white lines on the ground, as you need to cross two or three parking lots. You'll also have two or three traffic lights to cross. All in all it takes about 15-20 mins.

The "north" way is better for terminals B and C, and it's a little shorter. The downside is that there is no proper trail, and that you need to cross the airport access road when there is a gap in traffic.