Okay so there are a number of you to respond to:
Yikes: this was for anyone. Not just older runners. Noakes booke indicates that aerobic capacity is something that anyone, old or young can develop and improve by running the least amount of miles for the distances you want to run. He, of course, comes from the ultra-marathon perspective. Arthur would say, training for a marathon gives you the endurance to train well for ANYTHING else. Snell, though a 800-1500 Olympic medalist ran marathons. he was with the leaders through 22 miles, and ran into problems. Sat down and drank liquids, several minutes later was cajoled into finishing and ran 2:40 ish, 3 weeks later he ran a 1:50 (mind you the WR was 1:45 at the time). So younger folks do BETTER on aerobic work.
SoCalSchube: I do not coach at a HS. I have the benefit of having only runners who want to succeed who ask me to assist them with their running. Allison who was 4th in state this past CC season, 5th at 1600 and 3rd at 3200, had a 800 PR of 2:27, she is not a speedster. However, she ran 18:17, 11:03 and 5:07 here at altitude (she was 13th at Footlocker MW in 18:13). Her 400 PR (in a relay) was 67. So we are not talking about runners like you have. Should they have the endurance, they would be even better at 5 km. Allison's ran with the boy's team to get "pushed". She also ran 38:30 at Bolder Boulder, as a "fun" outing. I do emphasize fun with running. Janna, could be running sub 11:40 at altitude, but she worked 24 hours a week, ran NOT 1 STEP between state cross and Feb 15.
Its optional: Snell was a 2 minute 800 runner out of HS. So when you say that your runners were not a Snell, I take exception. Lydiard correctly said "there are Olympic champions in every neighborhood. They just need to be trained". Your kids are no different. Get them out running, having fun, developing. You may be hindering a potential Olympic medalist by thinking they are not a Peter Snell. Peter would tell you, he would not have been an Olympian, let alone champion if Arthur had not encouraged him to train.
Newyork1:
First, I graduated from Jamaica HS in NY, so good on ya. Speed is not important. First you have to be able cover the time. As your endurance improves, you WILL run the distance faster. It is incumbent on us to do that. SO first, cover the time, then each time you run that time you will have improved and the speed will develop with it. Speed of the run does not matter. The first time Snell ran over the hilly 22 mile route that Arthur used, he came in well over 2:30 and cried. He eventually ran the route under 2:10.
hope those help. I am still asking my compatriots to find the articles about recruiting fast twitch fibers after running over 1:45. When then get the references to me, they will be posted here.
Good running,
Glenn