If you want a real marathon,
9 Beet Stretch is your boy. It may have the downside of being impossible though. Maybe in 2 or 3 sessions?
Although it's not technically one piece in any of them, Table of The Elements has a few box sets that would be good choices. Either of the early minimalism ones, the John Cale or the Tony Conrad ones, would be amazing, especially if paired with that 'Inside The Dream Syndicate Volume 1: Day Of Niagra' CD that was originally supposed to be part of the Cale one, or the whole 'Guitar Trio Is My Life' set...