Mental Endurance

I think when people talk about a particular shakuhachi piece as an endurance challenge, they are talking about mental rather than physical endurance. Assuming you are in a comfortable posture, shakuhachi isn't really that physically challenging. You're sitting still and blowing into a bamboo tube, usually with plenty of time between breaths.

Endurance, in this case, means finishing the task at hand, bringing your attention back to the music no matter how many times it tries to wander away. It means getting to the last note of a piece even if you have a million excuses for not doing so. It means ignoring the ringing phone, the thoughts of what you should be doing, your plans for tomorrow. It means letting go of your judgment that you're not ready to play the whole thing and JUST DOING IT.

This kind of discipline is one of the great lessons of shakuhachi, maybe more important than the music itself.