I too was inspired when I read this. Of course, much of our potential is limited by our genes. But we can still try to get the most out of whatever we were born with. When I was marathoning, the plan was to do at least three runs over 20 miles before each one. That was about as much as my body could take. Whitlock did 43 such training runs before his remarkable 3:54 at age 73!
He certainly has the body build for a long distance runner,a very slight frame. His mile time as a youth is good but not remarkable. A bigger man can just generate more horsepower (think Peter Snell) for the typical track distances, so genes certainly play a part.
I do wonder what his life style and nutrition is like though, he must not eat a whole lot, and it would seem like it would be high quality food.