Here’s my take. Lydiard’s principles for younger distance athletes should be cut in half or a quarter. There is no reason a kid from the ages of 11-14 should be doing more than 30-35 miles a week on the high end, and 15-18 should be around 40-60 miles a week on the high end. I think Joe Newton is the most succesful HS XC coach ever, but his boys when they hit college are usually lost from the face of the earth, because they are burnout. Most succesful HS runners fit this bill. I think you can still generate 14:30 runners @ 3 miles with 45-55 miles a week of training, I know you will not get a sub 14 with that mileage, but kids until they are about 15 do not have their VO2max affected by training much. The overall characteristics left are how fast can they run and how long can they sustain a good pace.
If I could get every kid on my middle school XC team to go out and 6-2-6-2-6 then follow the next week with 2-6-2-6-2 in the summer I would send my team or several individuals to state each year. It would make my job easier for sure come the start of the season, because I don’t have to worry about building a base over 3 weeks then have 5 weeks to anaerobically train them. I still leave myself about a week or 2 short from having the best possible times I could get from them.
On the flip side I believe heavily in the conjugate system principles, some of my XC runners play club soccer during the XC season and I have them go play soccer 2-3x a week. They still performed at the same level as their teammates come race day.
My belief is at younger ages kids should be encouraged to do as many sports as possible and not forced into them. When these kids get older they will gravitate to the sports they are better at. The area where I live and I assume like many others around the country is baseball/basketball/football world. Many of these programs are bad, where playing a sport that you and your teamates are bad at is encouraged instead of having drifted to the sport that you find the most success. My biggest beef is with travel league baseball and the 60 game seasons now. Which is more than most HS and Legion seasons combined and more than most Colleges. Those 60 games are about the same length as most Rookie League seasons in the Minors. With that many games there is absolutely no practice or training that benefits these kids, at age 11 their skill level stops expanding and progressing. Imagine a track athlete that competes 3x a week over the course of 120 days, what worth would they be at meet 20? They would be toast only halfway thru the season. Counter that with the track athlete that competes 1x week over 120 days, they would be at meet 16, and probably just off their best times and thankful the season is ending.