This high school at a team time trial have 4 runners under 4:30 in December, way before the season starts. All of them broke 5. They probably went way faster during the season too, because usually people don't train as hard in the off-season. I think Sean Brosnan was very right when he said "4:20 is not fast, it's just not fast" because your ability is all in the mind. Also, I was watching a Jack Daniels lecture on YouTube and he said that he interviewed a runner who ran 360 miles in a week. Anything is possible when you put your mind to it.
Mentality > Physicality