That is an interesting theory, but you are definitely wrong about the marathon part. I know a number of people who have harmed themselves by attempting a marathon as their first race, a couple permanently.
My concern about the harm of an unprepared person taking a 10-day class is different. Most of the people I know who went from doing nothing to trying a marathon are no longer runners. Either the difficulty and pain or the experience of failure were enough to dissuade them from taking up a regular running practice. If someone jumps into a 10-day retreat with no prior experience, I would be concerned about something similar happening to them.
My concern about the harm of an unprepared person taking a 10-day class is different. Most of the people I know who went from doing nothing to trying a marathon are no longer runners. Either the difficulty and pain or the experience of failure were enough to dissuade them from taking up a regular running practice. If someone jumps into a 10-day retreat with no prior experience, I would be concerned about something similar happening to them.