How often do you have a break when you practice playing the organ?
It's human nature to continue one enjoyable activity, such as organ playing for as long as you have time for it. We can get so deeply immersed in the process that we forget the world around us.
This type of focus is good except when we ignore the signals our bodies send to the brains, we can overextend ourselves.
Pushing yourself through some hardship is OK but we still need to relax and take some rest from time to time.
It's just like reading a fascinating book - we forget to breath and we forget to blink which is not good for our eyes.
I always recommend taking a short 5 minute break for every 30 minutes or so of practice.
Take a walk, drink a glass of water, and stretch. Then come back and practice some more if you wish.
If you rest before you're tired, you'll never get tired from this activity.