I take it you want an indoor pool for year-round swimming. Outdoor pools here are seasonal from Memorial Day to around Columbus Day.
I've seen the pool a the Lennox Club, and another property with an indoor pool is the Landsburg in DC (I used to live there
specifically for the pool). Both pools are roughly 80' long and about 4 lanes wide; not full size but doable for laps. Archstone Pentagon Row has an indoor pool but it's dinky; I think my bedroom is larger. I haven't seen the one at Concord.
Regarding Arlington County pools, they run three indoor pools year round and a seasonal outdoor pool. I've used the one at Washington-Lee High School; highly recommended.
Get in the Swim! : Parks, Recreation & Cultural Resources : Arlington, Virginia
I haven't seen the pool at the Arlington YMCA but they do list aquatics on their website.
Arlington Branch YMCA