I agree here and with Adam. I relate well with what Adam said. Another thing that helps my endurance is doing excersizes with "Chop Stix". They are weighted sticks that are used like the old pencil excersizes. After a good practice session in the morning before work, I will do these excersizes in the car on the way to the office. You continue to work the facial muscles, but give your lips a break. I feel like this has made a big difference in my endurance. I can do a fairly gruelling 2 - 2 1/2 hour rehearsal, and although I will be tired, I can complete it with gas left in the tank.