If you are ever visiting DC and get tired of running around the Mall, there is an alternative. I was staying in NW at the Fairmont on 24th and M, but there are numerous other hotels in that area. I found this on a lark, but it turns out if you go down M to about 27th or 28th and hang a left, you hit the C&O canal trail, which takes you through a hidden part of Georgetown. About a half mile down that trail (and just after you pass under Key Bridge) you can cross over to the Capitol Crescent Trail, which is a rails to trails 2 lane that runs along the Potomac River. Great place to run through the woods if you prefer that setting to the urban landmarks.
Ah, but you’ve only just scratched the surface…
To build off that, you can take the c&o up to the chain bridge, cross over the Potomac, and head back south on the Potomac Heritage trail for a solid 9 mile loop of trail running. Just be careful not to trip over the bags of trash from the Hispanic fishermen, and homeless guys… 
There are so many great runs in DC. Oh how I miss it. Now I’m in Chicago, which has only one place to run - the lake. Granted, the lake is great, but I miss the variety of runs available in DC.
You are right, and the further you go the better it gets. But like everyone else said, DC is amazing for running. I’ve lived there for 3 years and am still finding new runs, I love it.