Compare that to a 9-yr-old who gravitates toward [or is pushed into] classical music. An hour a day simply won't get it done - you're looking at 3-4 hours / day minimum of hard, repetitious work generally done on one's own.
I don't think this is a fair comparison - a kid practicing classical music 3-4 hours a day doesn't potentially hinder their physical growth.
My issue isn't so much the father is aggressively pushing running - I have an issue with any parent aggressively pushing anything. My issue with aggressively pushing running/mileage is that it could screw with the kid physically. If you want to put a time frame on it, a kid practicing soccer for an hour a day would be a lot, but that is an hour that includes standing around in line, listening about strategy, learning new skill sets. What other sports also provide is more than just straight forward and back, repetitive motion. They allow different muscle groups to work, again dispersing the "hour of practice".
I do not know what these parents are actually doing in terms of mileage, so I do not want to judge them specifically. But if a parent is encouraging a 9 year old to be running 50 miles a week and getting workouts/schedule in, my opinion is that is going to be too much of a physical strain on their growing/developing structure. I would prefer my son/daughter have a variety of activities that include all different muscle groups, and at most an hour a day of "organized" activity.