I've heard the same thing, but for cycling. However I wonder if that is not mostly for people who train much more than a normal person, like a pro or your ultra marathon friends.

I understood the idea was that by running / cycling mostly in zone 2, you can put in more distance / time as you can recover more easily. It would not be possible to bike 30 hours in a week in zone 3, you would trash yourself.

But for someone like me, biking like 6-8 hours a week, I could easily do a lot of higher intensity, recovery time is not a bottleneck, other obligations in life are. So I really wonder if that still applies if you're only running / biking a few hours per week?