how is endurance affecting you specifically?
is it pain of the lips? tired jaw or finger muscles? shortness of breath (esp. while marching)?
here`s an answer to each of those:
lip pain: bear this in mind - your lower lip is flesh, not some super-durable polymer designed to play perpetually, so understandably there will always be a limit and it shouldn`t be your goal eliminate that limit. If you`re playing until your lip begins to scream in pain, you`re playing too much. back off for a day and give it another go tomorrow. If you need to play for long periods (that is, beyond your point of pain), give yourself frequent breaks and either drinks cold water (swirling about your lip) or hold an ice cube nearby in your mouth, to reduce swelling.
tired jaw muscles - if you practice every day, this probably won`t be an issue, but if you can`t (or simply won`t) practice every day, then a good alternative to building endurance on non-practice days is chewing gum - both sides (but really you`d get a lot more outta that jaw-energy if you were practicing
tired/weak fingers - when i first started out w/ the clarinet, i used to have trouble with dexterity and strength in my pinky fingers... I never tackled the problem specifically, and eventually through practice it took care of itself, but I advise younger players today to make sure their clarinet section is in possesion of one of those finger exercisers that guitar players use - they`re cheap and frankly i can`t think of any other device put together specifically for toning your finger`s strength
shortness of breath - sorry but the answer to this is simple - excersise. unless you have some breathing medical condition (in which case i`d consult your doctor), the way to go is to just do something aerobic a couple times a week (which you should do whether you play an instrument or not). jogging, swimming, badminton... those are my fixes...
also before doing any marching band activity out on the field... hydrate! a lot of getting tired has to do w/ dehydration, and the heavy breathing required of playing a wind instrument doesn`t help!