For endurance, I usually suggest practice in the same way I would train to start running for a race. Start out practicing regularly in shorter sessions and see if you can build up the time you can play over time. It comes mostly with time and persistence and doesnít come quickly. If you find your muscles giving out quicker throughout your days practice, then you may not have given them enough time to recover. Donít be afraid to take a day off if you are feeling serious fatigue, as long as you get back into regular practice once you have recouped.
Another possibility is inefficient embouchure usage. If you find yourself using a lot of pressure, tension, or in some cases an odd embouchure it can take a toll on those muscles faster and wear them out. There are players who absolutely succeed with irrugular set ups, but they are in the minority.
If you have been consistent in your playing and you arenít aware of any major deficiencies in your set up, then I would suggest getting some advice from a pro. See if you can set up a lesson with someone who might give you a good perspective on your playing. Just like a doctor isnít always the best at diagnosing themselves, we as band directors can often miss things in our own playing even though we teach it all day to others. It never hurts to have an extra set of eyes/ears for playing.
I think of it the same as when you exercise any other muscle. Take it slow to start, use proper form, give your muscles an appropriate time to recover, be consistent in your training, and if you are still hitting a barrier after consistent training, consult a professional.
I rambled on a bit, but I hope it was helpful. Best of luck with your playing!