My son is 15.  He has played football for the past 4 years and recently took up boxing.  He started running on the treadmill to train for boxing and liked it.  I took him with me to the local trail, strapped the gps on his wrist and sent him out for 3 miles.  His first time out his pace was 7:17.  The second time, having a better feel for what he was up against he ran at 6:56 pace.  The trail is relatively flat.  He has run on the treadmill to get in shape for boxing but was not really focusing on improving speed.  He has elevated the treadmill some what but I wouldn't compare it to running hills on the road.  Anyway, with proper training I'm hoping that he could manage a sub 20 minute 5k and be able to compete on the HS cross country team his jr and sr year.  He is going to stick with football this year and then plans on focusing on boxing and running.  He plans on entering in local 5ks.  At about a 7:00 pace starting point, how much improvement can he expect?  What are some of the best training programs for teenage cross country runners?