I made a simple equation to help me understand the effects of training. Let’s say I run 10 miles in 60 minutes at an average HR of 140. Take the miles as a whole number 1000 divided by the time 60=16.66. Divide your heart rate by 16.66.
140 divided by 16.66=8.43. This number indicates the workload heart efficiency. The lower the number, the better. So your goal becomes lowering that final number with whatever workouts you think will work. If the number ticks up for a few days, back off. It’s an indication you are not absorbing work. I was not the fastest marathoner by any means but 99.9% of the time, I got what I trained for. For ref I ran the following.
Mar 2.37
50k 3:24
60k 4.10
90k 7:02
50 miles 5:49
I have cancer so will probably never run again but my equation worked for me.