How long of a long run for a HS sophomore?

8-10 miles sounds good to me.

Build up to 12, then build up to 16. One hour is not a long run for a high school boy. You have to look at what successful programs do, not average programs. Example: Arcadia boys 13 week summer program varsity runners top 1000 miles, underclassmen and JV run 800-900 miles, lowest level runners asked to run 650 miles. That means a freshman may log up to 50 miles a week. That's why the high shcool wins national championships.

So, it sounds like your son is doing somewhere in the neighbourhood of 40 miles per week on average. If his typical daily run is ~40 minutes right now, it should be very easy for him to increase one Saturday or Sunday run each week to an hour, or 8 miles, however you want to measure it.

I think it would be great for him to start doing this immediately, if his coach is alright with it (which it sounds like he would be if the varsity runners are already running longer). I would bet that your son would see the benefits during spring track season.

If he builds up to 8-10 mile "long" runs during the spring, he'll have more strength and endurance going into his summer base training. He could then use the summer to steadily increase his overall mileage and bump up the long run to more like 10-12 miles each week.

That would be my prescription, if he wants to make the leap to varsity and help his team do well. From my sophomore to the beginning of junior year, I went from training like your son is doing now, did more in the spring, peaked at 70+ miles a week in the summer and reduced that a little bit during the XC season, and I improved tremendously. The exact numbers are not important; what is important is the general principle that the easiest way for a lightly-trained 16-year-old to improve is to run some more miles.

The most important thing is to make sure that your son really loves to run and thoroughly buys in to what he is doing. If his goals are to run faster and score for the team, tell him that you think that's a great plan, and that running more / longer will be most of what gets him there. Then leave it to him. I think that, once he sees himself doing more and improving steadily, he won't even need any encouragement.

The more important training variable is not how many miles on the longest run, but how many long runs. Miles per month are for more important than how many are run on the longest day in a typical week. Similarly, miles per year are even more important. A team of 4000 mile per year high school runners will dominate a team of 2000 mile per year runners. The difference between two schools which both have 2000 mile per year distance runners, based on how long their long run of the week is, pales in comparison.

Want one very important number? Annual miles run.

9 miles for now, hes only a sophmore. Over the summer into junior xc season bump to 10-11 miles with 60 miles a week. I dont think there's any reason for a high schooler to go more than 12 miles on a long run.

Interestingly, no US HS record holder at 800/1600 or 3200m, male or female, ran anything close to 4000 annual miles in their record setting season.

Webb ran 1000 less annual miles than Ryun for the 3:53

Verbicaz, Fernandez ran ~2000 less annual miles than J. Nelson.

Rupp ran 2000 less annual miles than Lindgren for the 5000m record.

Cheserek is doing ~500 less annual miles than Rupp as he heads towards <13:37 and 2000 less annual miles than Chapa as he heads toward the 10000m record.

Think carefully about these long runs, about an hour is sufficient.

He will improve by his body maturing anyway, you and the coach know your son, is he capable of taking more on.