Here we go again. Again, Alan Web was exceptional swimmer before highschool and before dropping swimming in favor of running before his sophomore year. To quote his wife: "I know as a freshman he was less than a second from making US Juniors in almost all 5 events he swam during a weekend meet (and failed to qualify). The next year he dropped swimming and only ran.". Swimming is obviously aerobic event, not to mention the hours that swimmers put in from early age with early two a days workouts, etc. You might be more familiar with it given you triathlon background.
I do not think anyone can say what Fisher, Webb or whoever could have run their freshmen year if they weren't doing sports. If they were sedentary and not doing anything during pre-high school years, they now would not be where they are or were in high school. There are so many factors that influence athlete's capabilities and genetics are only one component. As someone has said before, it's very likely that most talented distance runner or sprinter in the world might be living sedentary life and not even thinking about sports. Or doing other sports.
To sum up, Reed Fischer is outstanding talent who has also improved greatly from high school days due to many factors among them would be:
1) Lack of serious training in high school
2) Lack of focus towards running in high school
3) Late or prolonged puberty
4) Better coaching/more focus in college/after college
5) Great durability
6) Great aerobic talent as well as threshold capacity. ie. fractional vo2 utilization
Not being too focused in high school is not a knock. At that level it's more important so that kids would enjoy the sport and hopefully would continue doing it throughout their lives, rather than seek best possible result regardless of consequences.