Chesapeake & Ohio Canal
The Chesapeake and Ohio Canal National Historical Park stretches 184 miles from Washington DC, through Maryland, and West Virginia, eventually ending in Cumberland, Maryland. The towpath is a wide, smooth multi-use trail that follows the Potomac River. Some of the original lift locks that raised the canal from sea level at Georgetown to 610 feet at Cumberland, MD are still in existence.
Note that sections of the C&O Canal are pretty remote and that during hunting season, care must be taken to ensure you do not end up with a few extra holes, prematurely ending your outing. The only time we’ve ever been shot at was along the tow path and we were not properly attired in blaze orange - presumably it was an accident, but we did not stick around to find out and we won’t be making that mistake again!