I think "if you haven't done the training" is the operative phrase. Similarly nobody could write a novel if they hadn't learned to write, or read enough, etc. If a healthy person wants to run a marathon they will be able to by a process of training.
The original metaphor is a bit off. Anyone who puts in the effort could write fifty thousand words on the same theme or plot. Likewise anyone could complete a marathon with enough training and effort. A better comparison would be writing a novel that people would pay to read, and completing a marathon so quickly that people would pay you to do it.
While everyone can do the amateur version of these tasks not everyone can do the professional certain.
>A better comparison would be writing a novel that people would pay to read, and completing a marathon so quickly that people would pay you to do it.
It's not merely a case of writing something that someone would pay to read, but writing something that someone would read at all. That's a considerably lower hurdle, but one that many authors never vault. Every day, the slush piles of publishers swell with lovingly-crafted novels that will, in all likelihood, never be read by anyone.
The parallel I might draw is with a singer so dreadfully tone deaf that their neighbours lodge a complaint, or a comedian so bad that they are booed off stage at an open-mic night. Most first-time novelists are precisely that awful - their work is not merely of insufficient quality to attract a paying audience, but actively repellent. Few budding authors are prepared for that level of disappointment.
The original metaphor is a bit off. Anyone who puts in the effort could write fifty thousand words on the same theme or plot. Likewise anyone could complete a marathon with enough training and effort. A better comparison would be writing a novel that people would pay to read, and completing a marathon so quickly that people would pay you to do it.
While everyone can do the amateur version of these tasks not everyone can do the professional certain.