Pitcher 17 experience is pretty common I think. Same situation with my son. I know a friend was a pro who never threw over 180 feet and could sit 92-93. I have watched Zito toss out to 300 + feet, climb on the bump and throw 84. It really depends.
So, a guy can generate the arm speed to throw the ball that far but not necessary coordinate his mechanics to take advantage of it. Longtoss is great IMHO to condition the arm and as a part of arm care. The point of throwing for distance is to measure the effort of the throws, it gives immediate and real feed back. A person could get all of the same benefits throwing into a net at 20 feet. The hard part is getting real feedback that way.