I was back in DC this week for work. It really is an easy city to ride in if you are visiting for work. I stayed in the Westin Arlington Gateway, which was 2 blocks from 2 different bike paths.
The weather was so nice, I rode before work and after work. The Custis trail goes west next to a stream the whole way. You can jut off on short singletrack too…. very short, but dirt is dirt.
The trail in the morning was pretty empty. 
And you had to do one pretty deep stream crossing.
The hotel was nice, the gym was nice, 1 bike, 1 elliptical and 5 treadmills. Two benches and weights… The room was nice, had a big flat panel tv, nice view, wireless and wired internet. No soda machines though…