Keep in mind that NFL players train for short bursts, and value size, explosiveness, agility, and strength far more than cardiovascular endurance.
The average NFL player is approximately 6'3" 245 pounds. If you were to exclude lineman I'd say that number would go down to about 6'1" 220. And excluding linemen, that's a LEAN 220. Plus the weight and resistance of pads.
The cardiovascular demand at of a 6'1" 220 pound man is huge compared to that of an average pro soccer player at 5'10" 160. Who have virtually no value on size or strength. But a high value on cardiovascular endurance. They do place a high value on speed, but it's a different kind of speed.
A lot of Europeans and American Foot Fairies talk about football players like they're slow and out of shape. But I'd love to see even 6'1"(who is only 160) Christiano Ronaldo try to make it through an NFL practice.
Soccer is mostly standing around anyway, 18 guys on the field 2-3 chasing the ball, the rest standing around waiting. I once read that some soccer players run 4 miles per game. 4 miles in 90 minutes, that's only 2-3 miles per hour, a slow jog. An NFL offense runs about 70 plays per game. So 70 bursts at full speed(much faster than 3 mph, probably like 20mph for most of the DBs and WRs) and it's not just running. Blocking, pushing another big, amazing athlete for 5-7 seconds, or beating the press then running a route is by no means easy and has a very heavy anaerobic demand as well, something soccer lacks entirely.
You have to be at full speed, every single play.
The NFL has the best athletes in the world on display. We have 350 pound men running 5 second 40 yard dashes. Do you know how fast that is? Hint: faster than 90% on here.
I think this video speaks volumes to the NFL athlete compared to the average guy:
YouTube - Broadcast Yourself.