I think you listen to your body as well as looking at the numbers. In Eddy Merckx's day, before HRMs and powermeters, they trained on RPE and gearing. So you'd go out for three or four hours on (say) an 80 inch gear. Merckx did a lot of training being paced by a guy on a motorbike. It wasn't as scientific, but it got him pretty fit.

The more experienced one is, the more RPE can be useful. But even after all these years (I'm only five years younger than you) there are still occasions when I feel I'm working hard but then glance at the HRM to find I'm barely at tempo, and others when I feel I'm cruising but find I'm approaching threshold. So I think the combination is helpful.