I go mostly by perceived exertion. If I plan to ride longer than 2 hours, I start easier than usual and wait 30-60 minutes to decide whether to increase my effort.
On good days heart rate is a reliable indicator for me. But occasionally I take a small dose beta blocker for migraine, which can throw my heart rate way off for 2-3 days. Usually I set my bike computer to sound an audible alarm when I exceed 160 bpm. But after taking a beta blocker it's impossible for me to even reach 160 bpm. So if I can usually hold 130 bpm almost indefinitely, and 140-150 bpm for at least 2 hours, I need to mentally recalculate 20 bpm slower for the beta blocker. A maximum effort would be around 140 bpm.
And while a normal zone 2 effort for me might be under 130 bpm, after a beta blocker a 130 bpm effort feels like tempo effort or harder.
I had to take a low dose beta blocker twice last week, so it'll be another day or two before my heart rate is reliable for training again.
Meanwhile I'm just taking an easy week, judging my perceived effort based on whether I can carry on a conversation (harder to do since I've skipped group rides for months due to the pandemic) or singing to myself to guesstimate my effort.
And as the summer heat increases I need to compensate by easing the perceived effort even more. So even if I could risk a group ride, I wouldn't try to match their pace if they wanted to do a tempo ride based on the age range of 30-50something. At age 62 I can't match the tempo effort of younger folks for more than a couple of hours, especially in the heat. So even before the pandemic when I did group rides I'd often bail out after an hour or two and go my own pace.
I know when I've guesstimated my effort correctly for a metric or full century when I finish with my butt being the main concern. If I'm totally exhausted to the point that I don't even care how much my a$$ hurts, I've pushed too hard. Unless I'm aiming for a personal record time, there's no point in pushing that hard over distance in summer.
I know a fellow in his 50s who rides several centuries a year. A couple of years ago he did about two centuries a week for a full year. I've ridden with him and he maintains a steady effort, no sprints, doesn't hammer up climbs, and takes lots of short rest breaks -- long enough to snap a few photos -- and a longer break for lunch midway. And even that fairly leisurely approach for him still feels like work to me. That's how much difference a few years in age makes.