Preparing: Get the $10 "Official Guide to the C&O Canal and the Great Allegheny Passage." It has a mile-by-mile guide, maps, planning advice, the trails' history, and directories for each town, with lodging options, restaurants and services.
Starting Point: Cumberland, Md., is probably the best place to start. The trailhead has free parking, and Amtrak's walk-on bike service is available in either direction.
One-way options: Van shuttles, listed in the guidebook, offer options for one-way bike trips on the two trails.
Where to stay: Camping and B&Bs are available along the 335-mile trail system, but make reservations in advance.