On Physiological Testing and the Runner

I was reading awhile back where physiological testing was being done on some elite U.S. distance runners. The hope was that it would yield info that would highlight strengths, as well as the areas that needed to be worked on in order for the athletes to achieve optimal performance.
Decades ago former world record holder in the marathon(for 12 years),Derek Clayton, offered the following in response to what he thought about the extensive testing he had completed under the supervision of exercise physiologist Dr. David Costill.
According to Clayton, his test results were mostly unremarkable for a world record holder.He commented that he was happy that he hadn't been tested early in his career and been given those findings.
 He said: "Being tested would have eliminated the elements of the unknown. It would set limits that may only exist on machines that measure physiology rather than psychology.It is what a runner thinks he or she can do that creates success."
So true!
Derek closes with this statement that all of us should keep in mind, "Natural ability and determination are must haves,however, of all factors involved in distance running,I would say that the most important is determination."
Where most give up,successful runners and champions persevere.