What's Your Favorite Trail Inside the Beltway?

I know there are quite a few iRunFar readers inside the beltway. As many of these beltway insiders will be stopping by for iRunFar’s Massanutten 100 coverage this weekend, it seems like an ideal time to ask:

What are your favorite trails within the Capitol Beltway and why?

While I know many of the local trails, I’d love to learn of more. (Just last week I learned of the excellent Gulf Branch access trail to the Potomac Heritage Trail.) I’m sure other local iRunFar readers as well as future tourists or business travelers would also love to know more about the trails the area offers.

I’m also writing a article on the area’s trails and would like to hear people’s thoughts on the DC area trail systems. [I will ask for permission before quoting anyone.]

With that, it’s off to the Massanuttens!