I live outside the Beltway and have no safe route to ride into DC so I put my bike into the car and drive down to Rock Creek Park. From there I go through the park, usually going up the hill on Ross Drive instead of taking the "water level route" on Beach Drive. When I get downtown I go along West Potomac Park, cross under the 14th Street Bridge and ride along the waterfront on Maine Avenue to 7th Street Southwest to the office. Sometimes in the evening when I want a longer ride back, I cut through Georgetown and up the Georgetown Branch trail and Capital Crescent through Bethesda and then back down into Rock Creek Park from the North. I wish they'd re-pave the trail in Rock Creek Park....it's in terrible shape.
I'm 50 something and hope that I can continue to bike commuter half-time until I retire.
Several years ago when commuting in early one morning I passed President Clinton out jogging with his entourage of Secret Service men on the Mall across from the Smithsonian.