Do you ever wonder where great endurance horses come from?

A couple of my favorites are Heraldic and MONK.  Are horses of this caliber due to careful selective breeding, expert training programs, or a lucky spin on the roulette wheel?  I was looking at Heralidic's race record this evening.  All I can say is amazing.  I don't think I've look at Monk's total record, but I know it's good, VERY good.  What makes these and other incredible endurance athletes the flawless running machines that they are?  The drive and heart of a horse will only take it as far as the skeletal/muscular systems will allow before breakdown.  Yet there are a certain few horses that have it all...that take our breath away, that pull the emotion from us in understanding of what it is they do, and do so well.  I hope Monk rocks the Tevis this year.  Can't wait to vicariously watch the ponies run ☺