Excellent write up on heart rate training. I got introduced to this concept in 2019 through a YouTube video posted by Floris Gierman. I have been using the Maffetone approach ever since. It can be frustrating in the beginning but if you stick to it, it will bear fruit. When I started, I couldn’t run any distance (even a few yards) without my heart rate creeping up into the 150s and I was trying to keep it in the low 130s. But three years into it, my pace has moved from the 13s to 9:30 for the same heartrate. So, in essence with this approach, you don‘t train for pace but you let pace come to you. That said, I think this is better suited for adults in mid-life and beyond rather than kids and younger adults. Where I find this most useful is to define effort. As you point out, pace, heartrate and RPE are the common ways people use to assess effort. RPE is the best approach but takes a lot of experience and an ability to be brutally honest with oneself to be helpful. Heartrate provides an objective alternative to RPE. Pace is the variable used by most runners to assess effort but unfortunately this is the worst option to use. A pace that feels easy in 60deg F, 30% humidity, flat terrain will feel brutally hard in a hilly terrain on a sunny 75degF, 65% humidity day. It is also too sensitive to your sleep quality, hydration, state of health, state of training (over trained vs well rested). So, while pace is a place to start this journey, the sooner you develop an understanding of RPE the better for your training. One other point I want to add based on what I have heard from Jack Daniels (Specificity guru whose training fundamentals everyone uses in their training program) - when starting low HR training, in the initial phase you will be forced to slow down a lot to keep HR down but if your pace slows down so much that you can’t maintain good running mechanics, you will be better served doing run-walks where your mechanics during the run part does not suffer. Just my two cents. This is one of my favorite running topics and I have a lot of opinions about this but this is a very good starting point.

I used to wear a hydration pack. Works great especially for the really long runs. However, now that live races are happening, I am trying to learn to not depend on hydration packs. I hand carry a water bottle, and leave extra bottles along the route if necessary and simulate a race aid station experience 😀