Pace at heart rate is the only metric I trust

Mileage is an input. It tells you what you did, not what changed. If I run 60 km a week for two months and my easy pace hasn't moved, I did volume, not training.

The number I chart instead is average pace at 145 bpm on flat routes in similar weather. When that line trends down, fitness is real.

It's noisy. Heat, sleep, and caffeine all shift it by several seconds per kilometer. That's fine — I look at four-week rolling averages, not single runs.

This is also why I built a log that computes it automatically. Any metric I have to calculate manually is a metric I stop tracking by week three.