It's far more luck than anything else.
Is the term fundamental attribution bias? I can't remember, it's something like that -- the tendency we have to overestimate the role our behavior plays in a situation and underestimate external factors. You can run all the marathons you like and micromanage every fucking nutrient that enters your body, but if you come across the wrong pathogen, or have a gene that's just a little wonky, you're fucked.
ETA: Fundamental Attribution Error -- I was close!