20 is much too long for high schoolers. I know some high school runners who successfully did big miles and improved drastically, but they kept their long runs under control. I think the long run is one area that is very easy to go overboard and hurt yourself on. I have a theory that it becomes much easier to do damage to your body once you get close to 20 miles because your muscles are just so tired.
I'm not malmo, but on his "manifesto" here:
http://pih.bc.ca/malmosmanifesto.html
He has these "rules" for training:
1. Run twice a day, as many days as you can. Hopefully five, six or seven days a week.
2. Run more. How much? I dunno. You figure it out, but find out for yourself.
3. Run it faster.
4. Love running and LOVE racing.
5. Stay focused.
6. Set goals and don't be afraid to fail.
7. Listen to your body and don't be afraid to rest.
8. Compete WITH your comrades in sweat - never AGAINST them.
9. Smile a lot.
I think they are great suggestions. You can't tell a high school runner "you must run X miles per week to win a state title." How the hell do you know how much someone else needs to run? The ONLY way is to find out for yourself.
I am of the persuasion that the long run is important, but (for those NOT training for the marathon) should be capped at a maximum of about two hours, less for younger runners. As many posters have mentioned, the most a high schooler really needs to do is 15 every once in a while. 13 is a good distance, because at a decent pace, it works out to just about an hour and a half. A good ROUGH GUIDE to the length of your long run is "1.5x the daily average of your mileage in singles"