Except (unless they've built new paths since I last checked) you can't realistically walk to the airport either. The only non-limited-access highway entrance is on the north side of the airport, so unless GP lives in Ashburn, you'll have to walk around the airport first (and if you're east, that means finding a decent crossing for Route 28...). Then you have such pedestrian friendly places as this[1].

1: https://www.google.com/maps/@38.9768706,-77.4451826,3a,75y,2...