re: Explain it to me like I’m 5 - Running edition

To measure, I think this is similar to what you're saying: Retest a set distance periodically (5K, 10K, etc.) at a given HR cap. If you can run a 5K in 30 minutes with a HR cap of 135 today, then in 3 months see if you can run the same course with the same HR cap quicker than before. There are more variables to running outside like temp/humidity, course, etc. It's probably the easiest to measure on a stationary bike. I used to train a lot on an air bike for 30 mins at a set HR cap (135). I would just repeat that workout every few weeks and see if I could end up accumulating more milage in 30 mins at that same HR cap.