My school we ran at NTN my last 2 years, and have sent 2 teams since I graduated, with an individual sent on the years the team didn't make it.
Here's my take on it- it does start with having the numbers, I come from a team with well over 100 kids (3000+ 9-12 grade). More opportunities for talent, let alone we did know of a few very talented guys that played soccer or just didn't want to run.
The year before we made it, we had been ranked in the top 10 in our region (back before a regional meet was held). We realized we could be good the next year.
Next season came around, and we had 2 guys that went from being mid 17 5k'ers a year ago to 1620/30 types. 2 of our top guys improved a good amount as well and were consistent top contenders in most meets. We had the thought, "that NTN thing.. that sounds pretty cool, lets win state, and then lets try to get to nationals and see how good we really are." Well we won state, made it to nationals, and got handily tossed, finished further down than we thought we would.
Senior year we had the same fire, we were gearing up for nationals all year, made it there.
The following 2 years after I graduated my school sent teams, successfully finishing higher up each year. My take on it is this- we already had a reputable program, NTN allowed us to take it to a whole new level. The young guys on the team saw what we had done, had listened to all our stories, watched us race, witnessed us come home with heaps of free gear etc. and it clicked, "if they can do it we can do it, lets continue to build our programs legacy and earn those experiences and tell those stories ourselves."