This is a short story I read a long time ago, almost certainly before 2000 and likely before 1990.
I don't recall if it was in a magazine or in a book/anthology.
It was probably published a decade or more before I read it, because technology is not present in anything beyond about mid-1970s levels.
The plot remains very strongly with me though:
An exactly average man decides one day to buy, from an ad in the back of the yellow pages or newspaper classifieds or similar, a set of lessons that claim to impart a particular skill.
His friend advises against it, claiming they don't work very well, and are a waste of money.
But because he is the exactly average man, average in all respects, the lessons work extraordinarily well, and he becomes an expert/virtuoso/etc.
So he begins to get all the lessons and become extraordinary talented.
Some of the lessons include piano lessons and weight lifting/body building/health improvements.
I believe it ends in disaster for the average man, a cautionary tale about excesses in any form.
For the life of me I cannot Google this now, to share with a friend who claims to be "very average".