They train for 5 hours during the off-season. It's not all at once though. That bag of candy is getting burned off (which I guess is your point anyway)
Then of course they're the elite of the elite fast twitch athletes. Roughly speaking, there were 1,083,308 high school football players competing, and eventually 251 made it to pro. After simple calculation, we can get that the percentage of student-athletes going pro is approximately 0.023%.
Again, they're not the top 1%, or even the top 0.1%, but the friggin top .023%
ie you can round up 1000 athletes that fall into the top 1% of genetic freaks and only 23 of those in that group will be NFL worthy.