Over the summer, I experimented with walk breaks to keep the heart rate low while running. The problem was, my periods of running never got beyond a minute or two at the low heart rate.
I think the issue with that type of training is that you never force the body to adapt. It’s only when you run continuously for a while that the you really have an incentive to lower the heart rate.
If humans evolved as endurance running then the primary objective of a run is to endure. The problem then becomes “how do I keep running”, rather than “how do I run faster”. It is an optimization problem where the goal is to keep running and things like keeping the heart rate low are a means to that end.
Now I run continuously in training, but I make it a “game” and try to do things that lower the heart rate while running. For example, I might relax the shoulders, shorten the stride, etc.
I believe John Douillard wrote about similar things when he set up the goal of training sessions as who could run the fastest but also at the lowest heart rate.