I'm nearing 40, been back running for about 5 years. I have routinely run into some injuries and often need to scale back. But for almost the last 12 months my training has been really simple. I've worked up from 40 mpw to holding 60-70 mpw week all summer (might try to bump up again this fall).
The vast majority of my runs are easy, but not every single one of them. Like you, I follow HR. Not religiously. And I don't walk on steep hills just to prevent a spike. But I run in the 130s for well over 90 percent of my runs and it is easy and enjoyable. My max is in the mid to high 180s.
Most weeks are all easy runs between 60 and 75 minutes. One to two times a week I'll add a 30 minute double. About every other week I get a 90+ minute run, sometimes 2 hours.
I probably end up doing "workouts" about twice a month. I say "workouts" because they are just not that demanding, and I do not plan them in advance. My favorites are going to the track to run some 200s on/200s off. I do anywhere from 8 to 20+ depending on how I feel that day. The ons are around 5k pace or faster, and the offs are just easy running. Other workouts will be a run where I feel good and tell myself I'm just going to pick it up x number of times over the rest of the run. Maybe it's 3 times, maybe it's 7. Doesn't matter. I just run faster for however long I want and then jog recover for however long I want. The last type of workout I do is just a progression run. So on those days when you're just feeling good, maybe you push it up a hill and feel you could just roll on, well, I let it roll. Just get into a rhythm and cruise, sometimes picking it up more, sometimes just holding it. Again, I don't think it matters. And I never look at my paces until after a run. The only thing on the watch is time spent running and current HR.
And I don't think those faster sessions are all that important. For me the key is enjoying the process and having fun with it. I am also seeing some nice progress. Subjectively my body feels better, my legs look vein-ier, leaner, and feel harder. I also feel much more fit. I haven't tested my fitness in over 6 months, but my paces at the same HR continue to drop (even in the humid heat of the summer, which surprises me). And when I did test my paces I dropped my 10K from 39 to 37 and my half from 1:24 to just barely under 1:20. I think I could do better than both of those times right now, and I might try to do some small local time trials or something this fall.
I'm well aware this training would not be fun for many people. But for those who like it, or are curious about it, I think it has a lot of benefits. If you want some further reading on this I recommend Joe Henderson's old book Long Slow Distance. It used to be free on his site, but that's down now. You can pick it up for a couple bucks on Amazon. I think it's worth it. Over the last year I've read it 3 or 4 times, always getting a little more insight into this different approach. And gaining a little confidence to stick with it, too.