I can understand the difficulties with trying to find suitable venues that result in courses that turn out to be standardized distances, and also the difficulty, and expense, of accurately measuring such long courses.
But I can't understand why the already-laid-out courses can't be measured with Google Earth and those approximate distances be advertised. Advertising the run as a marathon when it takes only a few minutes on Google Earth to determine it is well short of a marathon is not right in my view.