Regan National Air Port is the best choice--- it's

connected to Metro (DC subway). Stay in the hotels along the metro rails ( Red line Bethesda and Rockvile area is the best: safe and not too expensive). You can take metro to DC downtown. When visiting National mall ( where White House, Capital and most musuems locate), you can take DC hop-on hop-oof trolley between sites to avoid long walk. See this: http://www.viator.com/tours/Washington-DC/Washington-DC-Hop-on-Hop-off-Trolley-Tour/d657-5046WAS_OTT
Good luck.