I had to.
A few days ago I came in to the church to practice at 8:30 in the morning, started playing my piece and at about 9:15 a couple of ladies climbed up the organ balcony.
They were all clad in black and I understood there would be a funeral soon. In fact, they told me the service would start at 1 pm.
It turns out that the doctor of our former President Valdas Adamkus had died. What's more is that they kindly asked me to play the organ until that. Oh, and they said, the former President with his wife would come at noon.
That's 3.5 hours of non stop music...
No big deal? Maybe. Maybe not.
At first I sight-read a lot of Bach's music. Then got tired and started to improvise in the style of Bach. Then got tired even more and started to improvise in my own style, all the while conscious about Mr. President and the all important guests who were gathering.
Of course, not for a second I could play something loud and something too fast. The music had to fit the occasion but I thought to make it more serene.
Oh, and by the way, it all had to make sense musically, too.
I'm telling you this in order not to brag. I know what I can do and what I can't yet do.
I'm telling you this in hope that you will be prepared, if somebody asked you to do this in the morning one day.
To play the organ for 3.5 hours non stop, either you should be an excellent sight-reader or learn to improvise.
Because there are not too many people who can keep that much of mastered repertoire under one's fingers and feet.