After not playing my almost 100 year old Piston Thumb Valve Schmidt Horn for well over 40 years from graduating Manhattan School of Music, and now involved with the Queens Symphonic Band,I have developed my embouchure somewhat again...but...my lip endurance is short lived..
How can I develop a longer lip endurance?
Phil Myers' answer:
My endurance routine involves only playing for five or ten seconds at a time immediately followed by an equal amount of rest.
So, five seconds on, five seconds off, etc. I find this builds my endurance much faster than playing straight through for a minute or two. That just tends to tear me down.