Okay this is my next dream. Now that I am living in Arlington I am surrounded by an amazing network of within the city trails. Some dirt, some gravel/crushed stone, some paved bike path. My biggest impression is that they are everywhere!
Some fo the trails I have explored completely in the last month include:
W&OD- paved- rails to trails- 45miles
Potomaac Heritage Trail- dirt- rocky nasty-18 miles
Capital Crescent Trail- paved-13 miles
Mt Vernon Trail- paved-18 miles
4 mile- run Trail -paved-7.5 miles
Custis Trail- paved- 4 miles
Bluemont Connector- paved- 2 miles
The above culminated with yesterdays cool run. Here from Garmin. Its called the Arlington triangle loop and it took me 2:39 to cover these 18 miles. I did get a little lost where the trails don't quite connect, but they are currently pooring concrete and bulldozing to make them do so as I type.
Previously I have run a series of connecting trails linking rock creek park to battery kimbal and grover archibald allowing me to run dirt from walter Reed's 16th street entrance all the way to fletchers boathouse and the C&O Canal. As well as running the bike path system thru rock creek- which is 20 miles long
Other area trails include the grandaddy of them all the C&O canal towpath = 184 miles
A great resource is linked here
My plan is to one day lay out a route that allows a completely urban trail ultra of 50+ miles. With all the above choices it shouldn't be to hard! It looks like you could start/finish at Fletchers boathouse and do a figure 8 loop into rock creek park connectinginto the crescent trail run south to the C& O then into the Arlington triangle loop and it'd be close to 50. hmmmmmmm