If you talk to good coaches or ask Dr. San Millàn directly, I think they will agree that end-to-end Z2 (or sub-LT1) is the intention, not necessarily the result, for endurance rides outdoors by regular shmoes. They will also tell you that the most important number is the RPE and not to get too hung up on power zones. I have heard San Millàn say he rides without power or HR because his RPE is so well calibrated. I am trying to get there too.