Here's how I look at it. You know how people say 'Don't let dogs eat chocolate'? Well, in the 60s and even 70s my dog and the dogs at all my friends' houses ate it. If we ate it, the dog licked the plates/finished the leftovers/got crumbs off the floor. It wasn't a problem then and isn't now.
Same thing with running long distances prior to age 24. It was common in the 70s and 80s but (with Alana Hadley and about 10% of Kenya's population as the exceptions) unheard of now. Interestingly, the period in which I was doing it coincides with the US being a global superpower in distance running.
I ran a marathon my first year of HS (and again the next) and I can tell you that going well under 3 hours didn't get me on the podium for 17-and-under. I'm referring to races for which no one traveled more than a hundred miles. Locals only, not on the national radar. A 2:54 scoring 5th place in your division (behind the girls 17-and under, too) in a race that had zero runners with any sponsorship would suggest that a lot of teenagers - 4 from my school often at a given race but a ton we didn't know - were doing the 10k up to marathon.
Of course, this isn't the case now and such behavior is vigorously opposed by the parent/coach crowd (my generation, ironically) as well as your peers. Training for and racing events longer than 5k prior to HS graduation offends many people greatly if the MB responses to me on such topics are any indication. Ditto longer than 10k if you have any NCAA elegibility. Only once you don't have the option of school-sanctioned running is road racing 'allowed' by the court of public opinion.
So, I would say F everyone and do it. I did, a whole lot of others did at the time, and Sammy Wanjiru did a few generations later. Doomesday prophets only need to show up at a race in which I easily win my age division to see that it is only a good move.