Chesapeake & Ohio Canal National Historical Park

The technical scramble along Bear Island — 1.7 miles of class-2/3 rock scrambling along the Potomac, returning on the flat towpath. Park at Great Falls Tavern ($20/vehicle, 7-day pass; America the Beautiful pass accepted).

Flat crushed-stone towpath along the Potomac. The iconic mid-Atlantic long-run / long-walk / dog-walk trail. Shaded and runnable in any weather. From Great Falls Tavern there is a $20/vehicle entrance fee; access from Old Anglers Inn or Carderock is free.

The 3,118-ft canal tunnel cut through Green Ridge, with a steep ridge-top bypass trail you can loop. Through the tunnel itself is flat and easy, but the bypass adds the climb that makes this a moderate outing. Bring a headlamp — the tunnel is pitch black and takes ~20 minutes to walk through. Closures recur for restoration work, ice, and bat-related seasonal restrictions.

Mile zero of the Great Allegheny Passage at Cumberland's Canal Place. Crushed-limestone rail-trail heading west toward Frostburg — gently graded uphill the whole way (the GAP climbs ~1,270 ft from Cumberland to Frostburg, capped at the railroad's 1.75% grade). The C&O Canal towpath ends here too — Cumberland is the GAP/C&O junction.

The final 11.5 miles of the C&O Canal towpath into Cumberland — Spring Gap (MP 173) to the western terminus at Canal Place (MP 184.5). Flat, shaded, riverside. Far quieter than the DC-end sections; you'll often have a mile of towpath to yourself.