I used a HRM for three years and found it puzzling. I consistently found myself running out of “zone”, regardless of workout or race. My max HR was 195 and I had real trouble running less than 140 bpm. And averaged 167bpm for a marathon. The HRM was most useful dialing in my LT pace and late in races, like the marathon, it was my “watchdog” making sure my HR didn’t drop under 160. It was helpful in monitoring my fitness or fatigue.
The HRM was a very useful tool but after applying that knowledge I found myself relying on it less and less in daily training. Though I usually used it for LT workouts and races.
Like the gentlemen who started this thread, I discovered that I need to run my marathon prep longs runs [last 2 months] faster then what was typically recommended. I also focused on my HR / condition and strenght by picking up the last 1-4 miles of those long runs. Running 2 minutes slower tham MRP is fine for building your base but since I wasn’t putting in more than 60mpw I needed to focus more on quality.
The one often overlooked or just forgotton aspect of marathon training is the accumulative effect that one year has on the next. My first year back to wanting to run a quality marathon the long runs just killed me. And two years later I was running faster and longer, but the recovery was MUCH better. You can’t become a good runner in one year, it takes many years.