Because 10 is such a nice even distance. As a 5K-infatuated, non-marathoner, 10 is my long run mile-count of choice. It's significantly longer than my average 7-miler, but not so long that I'm dying by the end (remember the run of death? I know Mer does!).
This route is great because it doesn't feel that long. The first 4 miles are a straight-shot out 16th street (rolling hills, but nothing crazy). Then by the time you're getting tired, it's back to familiar territory (the Rock Creek Park bike path). I first did this 10-miler with my Saturday morning Meet-Up group, DC Capital Striders. Interesting people and good conversations, followed by food and a day of lying around. Yes please.
Since that first time, I've done it twice, once again with DCCS, and once with a friend, who had no idea what he was getting himself into until it was too late (Me: Want to go for a run? Him: Sure! Later - Me: Let's do 10! Him: Ummm, ooookkkkk, I guess...). Don't worry, we're still friends.
Anywho, this route through Rock Creek Park is a great weekend run. The road is closed on Saturday and Sunday morning, so runners, bikers, and walkers take over.