Unlike when we were in our 20s & 30s
In our 60s + the body does not make fast physical improvements.
What upgraded physical improvement used to take a month to achieve ,
May (safely) take a year or more now, "if" you are in reasonable condition with good cholesterol levels, not much over weight, + other variables.

Me , I went to my PC doc, who sent me to a cardiologist after some
chest discomfort.
Tread mill stress test, blood work, & other test recommended by the docs

Now on a staton & low dose aspirin.
Doc said the days of "no pain no gain " are over.
Work up to new fitness levels slower.

I set the 520 to alarm at 170 Heart rate,
(Hit it yesterday & backed off)
Garmin 520 algorithm said "71 hr recoup time for next ride"
I road today, but a short 20 ride. (What does Garmin know )