They say Max HR divided by Resting HR times 15.2 approximates your VO2 max in mL/Kg.
I'm a few years older than OP. Prior to Long Covid, my Max HR was 183 and resting HR mid 40's. When Covid got me late last Fall, my resting HR was in the high 80's to low 90's and it took 5-6 months to come down to the mid 50's. According to an echocardiogram this Spring, my left ventricle was smaller by around 10-12%, The Cardiology people were happy with this improvement as my HR was now in the normal range, which according to their text books brought down off the mountain by Moses starts at 60 bpm. Of course, I saw the data differently as my snarky comment shows. My morning HRV also sucked.
As my metabolic fitness increases (mitochondria) along with stroke volume, my threshold HR actually drops despite making more power than when less fit.
Percentage difference between max HR and resting HR is supposedly a good proxy for percentage of VO2 max. So, 120 bpm for 60 year old with 180 bpm max and 60 bpm resting HR would be 50% and about equal to VT1 for moderately fit people whereas 60% would be quite fit aerobically. HRR is more related to VO2 max whereas the ability to exercise at higher and higher percentages of HRR (added to the resting rate) equates more to metabolic health or at least how I think of it.