"Epic" compositions worth giving a spin

Not concept albums, but a single LP side consisting of a work with some sort of unity/continuity.  Possibly with one or more recurring musical themes or ideas.  Could be vocal or pure instrumental and I would think in the 20-30 minute duration.  What made me think of this is "Supper's Ready," by Genesis, which I've listened to a few times recently.  It seems challenging to put together a piece that has repetition and variety such as this and I think they pulled it off nicely.  I especially like the concert version with the added visual interest of Peter Gabriel's costumes, gestures and mime routines, but for me this one works well with just the music.  Does anyone have a favorite one-side long-form piece to throw out?