Why do you train?

How long since you could do a sub 20 5k? Because HR is not a reliable indicator, especially as you get older. I've seen 198 BPM back in my road racing days, if I go over 170 BPM these days I feel like my heart is about to burst out of my ribcage. I'm averaging about one beat per minute less per year for cross races, based over the past 5 seasons or so.

Looking at Strava my fastest was 6 years ago, when I was 38.