> It’s pretty well known, in cycling at least, that hard interval training is super effective for untrained people or people coming back from a break, but the gains plateau relatively quickly, and the stress of doing this kind of work is hard to sustain for long periods.

This is 100% experience with both cycling and running, and something I worked out on my own early on, prior to the advent of smartphones and even talking to anyone who knew anything.

I enjoy sprinting, both running and cycling, but it’s mostly something I do to regain my endurance ability after a break. Two two weeks of high intensity interval training, and then I’m able to sustain moderate intensity jogging for 30+ plus again, or an hour cycling.