I have a regular distance run I use for training. Since I live and work in the DC area, and because my schedule is always wierd (owing to traffic, if nothing else) and because I just can't get out of bed to run before work, I tend to meet up with my RB after work and run in DC. Over the past couple of years, we've developed a route we particularly like. It's an "out and back", so we can choose pretty much any distance we want, up to about 24 miles. And there are water stops, though not enough to get away without carrying a bottle.
It's a very scenic park route that starts at the parking area for Gravely Point Park, near Reagan National Airport and heads north, picking up the Rock Creek Park trail. It's a paved path the whole way, sometimes beside the Potomac River, a bit around the tidal basin, and lots along Rock Creek. It is mostly flat, but there are some significant hills. And this time of year (early April), with the cherry blossoms coming out, it's downright beautiful. But it's nice all year round.
Here's the route: http://www.mapmyrun.com/run/united-states/dc/washington/457409879