Another vote for HRRV to monitor overreaching

Lots of riding just below LT1 and the aerobic threshold (80-90% of total training time) and a little bit (1-2 hours per week) around or just over VT2 is plenty to get fit.

At VT1, one can talk in several sentences but not talk endlessly but one can ride for hours and hours at this pace.

At VT2, talking is not possible. You're an air sucking machine. One can feel it (lactate of H+ as one prefers) in the legs. This is a hard pace and one can hold it for 30-60 minutes. Recovery from these kind of efforts for older riders isn't as good as younger riders. I notice a big difference in recovery from my mid 50's to my mid 60's.

One does not a HRM or PM to identify those inflections. Just do lots of riding just under VT1 and then do maybe an hour or two (10-20% of the weekly total) at a hard pace.