I've recently come to the same realization. I've always considered myself a more short-distance athlete, having been a sprinter and played more "burst" sports in high-school like football. My wife is a more long-distance runner. I've never understood why people like her were able to run for miles and miles without stopping. I could maybe keep up with her for the first mile, maybe 2, but then I was toast.

It's only recently when I learned about Zone 2 training that I came to understand that my training approach has always been backwards. I have always indexed my pace on what has always been a "respectable" mile time, on the assumption that if I run at that pace over and over, eventually I'll be able to go for longer at that pace. And that kind of is true, but not ever to the extent I can keep up with my wife.

Now, I am attempting to approach my running from a "time spent running > speed", and I'm finding that I indeed can run for 40+ minutes straight with no stopping, at a much lower pace than what I had been, and that it is much easier to make progress by running slowly for 40 minutes and gradually increase my pace over time than it is to run quickly for as long as I can and gradually increase the distance.