We're spoiled as hell in DC. We've got a national park (Rock Creek Park) inside the city that goes on for miles and is just gorgeous on its own, just steps from my house. Specifically, there's a paved road in there called Beach North that, during the weekends, is closed to traffic entirely and is flush with bicyclists and hikers. It's one of the best trails I know of on Earth, and last time I went out on it I did 25 miles, all the way into Maryland and it never stopped being just as fantastic. Here's a shot from when I was out there in September. I think I'll do that again tomorrow. Because I like to show everyone, Look how magnificent it is there
Rock creek is awesome. I stay at the Sheraton near there and have run that section of the trail a few times. It's very nice and it's a rarity for a city like DC to have something like that right downtown, for sure.