Here's a non-brilliant concept for improving endurance. Take that upcoming recital, or one of those strenuous brass concerts in which survival is the issue. Good practice involves just as much the right mental preparation as physical. The right kind of approach and mindset will produce the best results. It's not a matter of energy, but how it is used.
Look at that crazy guy speeding down the highway in the snow and ice! Just up ahead we'll likely find him stuck in the ditch. Having lost control, he is quickly sidelined. Look at that marathon runner way out in front of everybody immediately after the starting gun! We'll soon watch him huffing and puffing himself into premature exhaustion as everyone passes him by. Now listen to that audition contestant blasting his solo and all of his excerpts in the warm up room non stop! We'll soon see him trudging sadly back to where he came from. Each of these did not finish because of energy failure - not the lack of it, but the misuse of it.
Playing is an athletic event that requires pacing. Had the driver, the runner and the audition contestant been able to conserve their energy, they would have finished as planned. A tense mindset burns up too much energy, while a wise relaxed approach uses less of it and is more productive. The impetuous, the driven, and the frenzied usually defeat themselves well before the end of the day.
Suggestion: Drive to get there, not to be the first. Run to finish strong, not to impress everyone from the get-go. And play your best, but so as to be able to repeat the performance tomorrow. As you prepare, pretend that the event is actually twice as long, and pace yourself accordingly.