Pacers is a nice little running store on King Street about half way between the Metro and the Waterfront. Even better they have a running club that meets here, several days a week, I think on Tuesday and Thursday at 7pm as well as Saturday at 8ish am.
They sell a little of everything that you'd expect your neighborhood running store to sell, from a wide variety of shoes and orthotics to clothes, energy bars, strollers and more.
Pacers has locations in Alexandria, Arlington, Fairfax and Silver Spring, MD.
Biking or jogging in Old Town Alexandria? The city streets are rough cobblestone and there are lots of lights, not to mention those locals who roll through stop signs without looking for pedestrians. A much better bet is to head down to the waterfront and enjoy the Mount Vernon Trail.
The Mount Vernon Trail runs along the south and west bank of the Potomac from Roosevelt Island to Mount Vernon, a distance of some 18 miles. The trail is paved and almost perfectly flat for its entire length. In Old Town Alexandria the trail is at its worst as it runs along the water in places, but cuts around the back side of warehouses and along the city streets in others. The confusing stretch only lasts about a half mile, however, and once you leave town the trail becomes an excellent bike and running route. To the south of Old Town you'll pass under the Woodrow Wilson Bridge and the Jones Point Lighthouse. To the north you will run past some Egyptian-looking artifacts then the city power plant before entering Arlington.
There are also many other running trails in the DC/Northern Virgina area:
Hains Point is one of the most popular running locations I have found, even though it is a relatively uninspiring, flat, maybe 4.5 mile loop surrounded on all sides by water. I did this run once in December 2008 and the winds blowing across the open Potomac were fierce and cold. On most weekends this place is packed with runners, cyclists, and rollerbladers.
C&O Canal Towpath is a 185 mile long trail that follows the Potomac River from Georgetown through much of Maryland. The trail is on the original towpath, so it is a pretty wide, flat, dirt, trail.
The National Mall is where I log most of my running miles. It is about 2 miles end to end, and it connects to some of the trails along the river, including the Hains Point Loop and the route to the C&O Canal.
Roosevelt Island has about four or five miles of trails, mostly dirt, that circle the island. While it may not be the best place for your entire run, it makes a nice scenic detour off the Custis Trail.
The Martha Custis Trail parallels Interstate 66 and runs about four miles from the W&OD trail to the Potomac River. The trial is paved along the Potomac, and I assume for the rest of its length.