HIT training is how the human body is designed to function. We are not meant to be in constant motion for our entire existence, but periods of high intensive activity and then followed by rest.
When I was young, I had a three mile fartlek route and I would sprint each night. I would give my all through the route. I ended up being able to run half-marathon distances without any trouble whatsoever.
I’m 72 and have been a consistent runner-cyclist since my sophomore year in high school. While I enjoy long runs, I also mix in Special High Intensity Training, which are not fun - (thus my friends and I refer to them as S.H.I.T. sessions).
There are some guys on this forum who are my age and older, who are really fast. I’m not one of them.