I’m a millennial living in US (although non native English speaker) and had no idea.

i've never heard of it.

There are many things it seems GenZ is unaware of but it is very much a common, well-known expression for more serious people.

Does more serious equal older? I’m already too serious!

I know quite a few teenagers who are far more serious than my middle-aged self.

It's an uncommon expression then. The commenter might be British. Anyway, anyone reading this thread knows it now.

I recently met someone who uses the expression "country mile" all the time. Hadn't heard it before but it's cool. "That's more popular by a country mile" Country miles are obviously better than city miles. So now I'll probably end up using it.

A country mile is often hot and dusty and walked uphill both ways by your grandpa. Time stands still and the cicadas buzz. It’s bigger than a city mile even in how much imagination it takes to conjure.