Mountain's "Nantucket Sleighride" from Twin Peaks is an all time fave.
Cream "Live" Vol I+II have many fantastic, extended improvisations,
Vanilla Fudge "Break Song" from the Beat Goes On,
Pat Metheny's "As Falls Wichita...." , the whole album is like one cohesive piece,
Pharoah Sanders' earlier works are delightfully long and rather thoughtful,
dare I mention the 70's Miles Davis - he is not Prog, yet?
Is Shakti Prog, yet? I am not sure. Their first album starts with a mind blowing, long piece.
Rare Earth - In Concert has great, long tracks, another fave to me.
Have we got Al DiMeola in yet? John Abercrombie, Jaco Pastorious, Cannonball Adderley have nice, longer pieces, too.