I'm also 78. I have a power meter on my single, but not on our tandem. I ride with a Garmin on all my bikes and wear a HR strap-type transmitter on all my rides. I've done a lot of training, group rides, and brevets with these devices. HR is your most important thing to watch because it tells you what your physiology is doing, sort of the output end of the process. A PM is the input side of the process. Thus it's interesting to note the output results of the input under various conditions. What a PM is good for is doing intervals while holding a steady power and also noting maximum power during sprints and intervals. If your short term power goes up, you're getting stronger, duh. If your HR goes down over time on the same ride, you're getting fitter, duh. If your HR goes up at the same power on a longish ride, you're probably getting dehydrated. If your HR goes down at the same power during a ride, you probably need to consume some carbs.

Thus as others have said on both forums, for now just watch the two numbers and notice how and when they do and don't correlate. But as far as your personal goals for fitness and long life go, a PM doesn't add much. Mostly that's just a matter of going to the gym and lifting heavy while also riding lots. As you've noticed by now, just "riding lots" has a lot of crazies talking more detail and arguing about it than a sane person unacquainted with said detail would suspect. Welcome to power data!