In the interests of adding a bit of context:
My "running" is basically an extremely slowed-down version of Couch To 5K, because the steepness of the curve on the regular version would break me.
And I'm in no way sedentary, just 49, dyspraxic (so "learning how to run" is a thing), have weird fatigue issues, and have been avoiding running for my entire life. And want to have a bit of energy spare for things like climbing.
Because the demographic of this forum inevitably skews towards people who climb a lot and tend to be pretty strong, I think there's a tendency to overestimate what you think the average person should be able to "just do" without any specific training.
Even before you start to bring in people who are in some "not average" (and of course it's extremely common to be "not average" in ways like being middle-aged, having minor physical problems, etc. etc.).