Walking distance is a bit deceptive with that hotel, with any of the ones on that road, really, because Jeff Davis Hwy. is difficult to cross on foot. The nearest metro station is Crystal City, and there are two ways to enter the station, either by walking into the Crystal Gateway Marriott hotel and walking through the tunnel under Jeff Davis Hwy, or by walking south along the Hwy and crossing under it at
15th St., then down around several buildings to the outdoor station entrance on 18th St. Either way the walk is about a 1/2 mile. Jeff Davis is a very busy elevated highway spur right next to I-95; Crystal City is directly opposite Nat'l Airport. It does have good metro access once you reach the station, as it is served by both the Orange and Blue lines. Alternatively, during mall hours you can turn west on 15th street instead and go through the Fashion Center mall to the Pentagon City station instead, but only do that when the mall is open because the station is on the far opposite corner of the mall complex; having to walk AROUND that large mall makes the walk a whole lot longer than cutting through the middle of it. (Pentagon City station is a bit farther away than Crystal City, about 2/10 mile difference.)
I haven't stayed at the Americana, but I have stayed in Crystal City. The walk to the Metro can be quite windy outdoors because of the proximity of the highways and the tall buildings there. If it is really cold, really hot, or raining, you'll probably want to take the tunnel under the Marriott.