Running

But when runners say, “I trained in Zone 2 for a year and didn’t improve,” the issue is rarely a lack of patience or discipline. It’s usually a misunderstanding of how Zone 2 fits into the larger training picture.

Judging by this quote, I assume the author is referring to people who just train in zone two. Though, if you know there's a zone 2, it follows that you would know that there's other zones that you should be training in too.