My experience is very similar to Doggit's. In the spring of 1972, my college classmate and I were visiting graduate-level seminarians in Washington DC, and one of them arranged for us to go over and get a close-up view of the organ in the church across the street (since they found out I was a budding organist).
We went over, and the organist (music director, Mr. Michaud) took us up to the loft. We watched as he played the opening hymn, and then he explained the organ to us - the divisions, ranks, electro-pneumatic action, etc. Then he said, "I won't be in tomorrow, but you can get the key from the secretary and come up and practice if you want."
I was dumfounded at first, but I finally stammered out, "Okay, sure." Prior to this, my only organ experience was less than two years on a chapel's B-3 (that's as close to cussing as I'll get here) and a 3-manual Allen at the college. I never had a lesson, learning only from a book on CHORD organ. Anyway, the next day, we went up, I sat at the console, and yes, it was like a Piper Cub student pilot in the cockpit of a 747 or maybe a space shuttle, because this was the 4-manual Moller at the National Shrine in DC. I only used the presets that Mr. M. had prepared, and played for about an hour. It was heaven!