The Allman Brothers did a 22:56 version of 'Whipping Post' on At Fillmore East.

Led Zeppelin did a 26:52 version of 'Dazed and Confused' on The Song Remains the Same

Mike Oldfield's 'Tubular Bells' beats both if considered as one song. On the All Music Guide website they have it listed as part 1 (25:36) and part 2 (23:20) so I guess we can each decide how to tabulate it.