Yep, I was not excluding muscle memory when I said it had to do with muscle. It is true that breathing through the nose seems effortless (at least when resting)

Yep, I was not excluding muscle memory when I said it had to do with muscle. It is true that breathing through the nose seems effortless (at least when resting). The "reeducation" consisted in a few 30 minutes sessions (probably around 5, not sure) and I don't remember having done any conscious effort to breathe through my nose after that. It just seemed easy. That said, I still have to breathe through my mouth when running.