What do you mean by "walkable Arlington"? If you mean a more house-and-sidewalk residential neighborhood, then neither Pentagon City nor Crystal City meet that mark. And Crystal city is fairly walkable even before you account for the underground part.

They're both mixed-use urban developed areas with large towers, although Crystal City actually seems to have more of those. Both are in the same place, south of Interstate 395 and west of the airport (the Pentagon is just north of 395).

edit: and for reference Wikipedia says: "Crystal City includes offices of numerous defense contractors, the United States Department of Labor, the United States Marshals Service, and many satellite offices for The Pentagon. It is also the location of Ronald Reagan Washington National Airport."

All true, but I'd like to add that - after having stayed at Crystal City Doubletree - that hotel is full of service members and what appears to be higher ranking Pentagon staff.