BTW, if you try any of the HR tests in the various video and web tutorials, do it at home on a trainer, or in a gym with a coach and spin bike with all the nifty measuring doodads.
Because if you push to reach that max, guaranteed you'll be on the verge of passing out or puking. Anything short of that won't accurately test your maximum.
Found that out a couple of years ago when I was doing high intensity interval training outdoors on a nearby circuit that's popular locally with pros and serious amateurs. It's a 6-mile elongated, narrow oval loop with rollers on one side and long steady climb and downhill on the other. It's great for intervals.
But not so great in summer heat and humidity. Closest I've ever come to fainting and barfing in training or competition. Never experienced anything like it, even getting knocked down by body shots to the liver as an amateur boxer against more experienced guys with vicious left hooks.
So now I do my interval training indoors only. If I fall off the bike it's carpet on one side and a comfy sofa on the other. And well fed fluffy cats, soft to land on and probably won't eat my dead body. For a few days.
And first work your way up gradually. Sounds like you're already doing that.
And ditto the frustrations of losing some maximum potential with aging. I'm stronger than I was a couple of years ago, but, realistically, I'm getting close to my limit before age forces inevitable limitations. Every time I think I've come close to maxxing out my potential, I'll check Strava for guys my age or a little older, and I'm still generally the slowest of the best 60+ guys in my area. I finally managed a couple of 17 mph averages on my favorite 20 mile route this week. That's some improvement since 12 mph in 2015, 14 mph in 2016, then 15-16 in 2017-'18. But the other guys my age or a little older do that route at 18-20 mph.
On the other hand, if I had the money and could find an appropriate doctor and trainer for medically supervised juicing, I'd darned sure try EPO, steroids, the whole deal. Not for competition. Just curious to experience the effects and enjoy a brief period of rejuvenation before the inevitable.