Comment by wry_durian

I don't know which video it is either, but what I've found helpful is checking aerobic decoupling. If your heart rate is drifting upward significantly over the course of an hourlong easy run despite not increasing your pace or power, then the bottleneck is probably your aerobic engine. I don't track it daily because it's a noisy metric, but looking at the overall distribution is useful.

After that it's probably determining your LTHR accurately (don't just accept what your Garmin or Apple Watch says it is). This gives a better sense of how hard to push on your hard days, and it's also useful to see what % of HRmax your threshold lands at.