> I’m roughly around 40bpm at the moment, down from ~60bpm 10 years ago
To put that into perspective for other readers: I've been running for about 10 years also, but typically 2x a week (10k mid-week, 15~20k weekend), I have no real data on my heart rate from when I started, but at rest I'm now typically at 60bpm.
I measure almost daily due to medication and having a minor heart defect, and I have noticed that if I skip a week of running, it'll slowly go up, averaging at ~62bpm, but when I train for a (half) marathon I typically increase my distance a bit and try to train every other day (~3x/week) then my heart rate a rest goes down a bit to be consistently below 60bpm (58bpm avg).
40bpm is very low, for non-athletes this would be considered dangerously low, but I guess daily running at OPs distance would classify OP as an athlete. Also keep in mind that heart rate differs per person, some people just naturally have a low heartbeat.