By Mary Pickles: WQED’s new documentary, “The Great Ride,” will explore a destination on many bikers’ and hikers’ bucket lists — the entire 335 miles of the C&O Canal Towpath (Washington, D.C. to Cumberland, Md.) and the Great Allegheny Passage (Cumberland, Md. to Pittsburgh). The 60-minute film premieres at 8 p.m. March 14.