Come on, man, you know it is giftedness. The training only kicks it into action and it shows up as "natural talent".
Someone who has lots of MHC 1 fibers as a toddler can quickly convert a sizable portion of them to MHC 2a fibers with training (while MHC 2X fibers also shift towards 2a type). All of a sudden, that kid is a unusually talented sprinter! Same sort of thing could apply towards having a naturally high volume of sarcoplasmic reticulum, so you can release more Ca(2+) during explosive contractions.
Then there is the idea that someone could, with luck or training, switch on some genetic switch (epigenetic change) that would then endow the runner with outstanding endurance - like some rat experiments have demonstrated.