The ADC Washington Street Map—don't visit our nation's capitol without it.
"But," you ask, "where do you get the ADC map? What is it?"
The ADC Regional Bike Map is the best map (not just for cycling) of the DC metro area and will be of great help should the C&O towpath be muddy—as there are many additional routes off of the path illustrated. Almost any DC bike store should have them and they may also be available inside Amtrak's Union Station (Map)—get one at a newsstand while you wait for your bike and gear to be unloaded (You will have time!) at the baggage-claim.
Otherwise, grab a cabbie or policeman out in front of the station and inquire as to the closest bike store. If all else fails, head for the river a few blocks away and then turn right, there is a shared use path along the river which will take you past a bike shop (if your eyes are sharp) or directly to the towpath (along the river).
The route between Union station and the C&O.
Ask a local cyclist were the shop is and the shop can direct you to the HI-Washington DC hostel. The hostel is just a few blocks from "M Street" (Map) which parallels the towpath and river bike trail in the Georgetown area. Make reservations now for the hostel, it is nearly always fully booked for walk-ins.
Adrian recommends the Hilltop Hostel over the HI-AYH facility: Fun place; friendly and helpful staff; convenient location; Very bike friendly, hammocks out back, relaxed atmosphere, major transit station just outside, min. age 18—no grade-school field-trip groups, laundry.
You will have a good time. By the way, we used to sell the maps but I have run out.
P.S.: The Lonely Planet advises, "Bring your own wiretap."
Yours in Cycling,
North Road Bicycle Imports
PO Box 840
166 Court Square
Yanceyville, NC 27379