Yes you are what you train to be, so the first mistake people make in training young runners or swimmers for that matter is to pile on the miles. Build that big aerobic base and get them real slow! It is preferable to start with good sound running mechanics and get them comfortable running fast. Then gradually build up their ability to carry that speed. That does mean to imply that it is an either or proposition, but it should be a mix or a blend. The reason the US is struggling to produce distance runners is that during the prime years when speed can be developed they are out slogging on the roads, learning how to run slow and getting rewarded for it. In 1972 we had five boys, ninth graders, run from 4:50 down to 4:30 in the mile. None of those boys ran over five miles in any one run, they never ran twice a day, peak mileage was 30 miles for one of the boys. All the rest were around 25 miles. They all were good athletes; they had five days a week of vigorous physical education. They had fun and played other sports. It is not rocket science, it is common sense. Treat the runner like an athlete. Get them functionally fit, make sure they get familiar with all three planes of motion. If you do that they won’t get hurt, they will get faster and they will have a great experience. Remember keep it FUNdamental.