Rock Creek Park runs right through the city, so you are never far from decent training routes even right down town.

Tues/thurs evenings in the summer months you've got the Goon Ride that starts out at the north end of the Park. On weekends, you can find large group rides leaving some where in the park Sat (7:00 AM, 7:30 AM, 10:00 AM) and Sun (8:30, 10:00).