This is what I found on their website:
These you can/should be able to walk to the metro:
Comfort Inn Downtown DC/Convention Center(DC601)
1201 13th Street, NW
Washington, DC, US, 20005
The McPherson Square Metrorail station and Mt. Vernon Square Metrorail station are within three blocks, making it easy to visit all the area has to offer.
Comfort Inn Ballston(VA417)
1211 N. Glebe Rd.
Arlington, VA, US, 22201
just blocks from the MetroRail's Orange Line Ballston-MU station
These have a shuttle to a nearby metro that is a little bit further than walking distance
Comfort Inn & Suites Near Union Station(DC012)
1600 New York Ave. N.E.
Washington, DC, US, 20002
Offers courtesy shuttle to the metro
Comfort Inn Pentagon City(VA383)
2480 South Glebe Rd. Bldg. 2
Arlington, VA, US, 22206
Shuttle: free transportation to Reagan National Airport (DCA) and Pentagon City Metro Station / Fashion Center (metro is less than 2 miles away) (I believe this is blue line)
Comfort Inn Near Arlington Cemetery(VA090)
6111 Arlington Blvd.
Falls Church, VA, US, 22044
free scheduled shuttle service to the Ballston Metro station (orange line)
Also one on Van Dorn in Alexandria with free shuttle to Van Dorn Metro, Landmark in Alexandria on Duke street also offers shuttle to Van Dorn
I am less familiar with MD, but here are some on that side:
Comfort Inn Georgia Ave/DC Gateway(MD088)
7990 Georgia Avenue
Silver Spring, MD, US, 20910-4860
College Park location offers free shuttle to Greenbelt or college park metro
And I think that was it for the suites
Within a 10-minute walk from the hotel is the Silver Spring Metrorail station (Red Line); free transportation can be provided.
HTH!