Isn't it curious how your mind may wonder quite far from your present activity when you are playing organ music?
"I made a mistake in this measure. I wonder if my listeners heard it?"
"Oh boy, I swear I will never ever let myself play in public without having memorized (at least for myself) the pieces I have to perform".
"Finally, I got this difficult spot right. A few more lines to go."
These are all examples of what we may think of when we play.organ music. Notice though, that none of these thoughts actually concern with what we are doing here and now.
But with improvisation it's different. Unless you are so panicking that you basically freeze on the organ bench, when you are improvising, your thoughts are completely in the now.
You have to be focused enough that you notice what is happening right here and right now. You also have to be relaxed enough that you take what you notice, take whatever musical material is available to you at the moment, forget and forgive yourself that you actually don't know everything, and create an interesting musical story out of that.
You can go one step further - transfer the same feeling of focused relaxation when you play actual organ pieces as well.
That's the ideal state of mind - optimum performance and experience state which scientists call flow.