Great city to visit. Spent last weekend there and it was spectacular. Easy city to get around in even during the 20th anniversary of the million man march. In addition to walking, we logged 10 to 12 miles per day 4 days in a row, make use of the Metro, Uber and Lyft. All were great help to seeing so much in a short time.
Note - Arlington National Cemetery closes around 4:00, maybe 5:00 so if you want to see it go early afternoon at the latest. I highly recommend seeing most monuments in daylight and at night. Striking differences at most of them. I only got to see the FDR and Korean War Memorial at night and these two need to be visited in daylight IMHO. The White House is quite a sight daylight or night.
Hope you have a great time.