Playball2011, sorry I'm on my phone and don't know how to reply with quote from the mobile site lol.
For a general run down. Athletic performance regardless of explosive or endurance related requires the use of oxygen. Your ability to take oxygen out of the air and get it to muscle is measure by your Vo2 uptake. The higher this is the more oxygen your body can take in and use to power your body. The more oxygen the body has essentially the more efficient and powerful the contraction. Sooo distance running is the easiest way for an athlete to increase his or her Vo2 uptake. Fast pace distance running not light jogging. Now can it be done with just sprints. YES. A good trainer can do it with jumping jacks if they wanted to. However if you don't have access to one of these trainers (which IMO most HS athletes don't) then distance running is the best way to do it.
I would consider a well conditioned athlete anyone who's Vo2 uptake is near maximum threshold. A well trained one would be one who's base strength and conditioning is well rounded and defined. IMO too many athletes are trying to do sport or activity specific training when they haven't built up the base strong enough. Just because a pro is doing it doesn't mean a 16 year old should be doing it.