You and I are in the same boat when it comes to AeT. We’re old! We are not going to see much increase in HR as our AeT improves. At our age our HR reserve: The distance between resting HR and MaxHR is greatly reduced from our youth. I’ll use myself as an example. My resting HR is about 50bpm now. When I was in my 30s and training more and harder it was 40. Now I can’t handle nearly as much training load and I suspect that’s the biggest reason it is higher. The cardiac muscle is just not getting enough training stimulus. In my 30s my max HR was 185 (remember there is a huge interpersonal variation for maxHR) Now I would have to do some serious damage to hit 160 (my 35year old self had an AnT of 165). This is just one of the many benefits of getting older. It explains why we can’t run as fast or as far as we once did, let a lone jump as high. Now a days my AeT is typically about 130 (some especially good days It will be up at 135+ and on some days when I am tired it’ll be around 125. In my 30s my AeT was 155.
So, you might have gotten as much AeT HR change as you are going to get. But, much more importantly than HR is pace at AeT. You’ve seen significant changes in your pace and that is really what matters. That’s a fair measure of performance and isn’t that what we’re really after?
These tests are not infallible because we are not machines and it is hard to recreate the same conditions each time. You can see the same thing even with laboratory gas exchange tests. As they say; “results may vary”. The HR drift test seems to correlate very well with both GET and blood lactate tests when we have tested them back to back.
In short I think you can forego more HR drift tests. In stead I would continue to watch and relish the improvement in pace and effort at 120HR. Here is whatI recommend: Do your own little AeT time trial. Find a nice long hill or course you use regularly and test yourself there using 120 as the upper limit. Find some convenient landmarks at 30-45-60 min and note your improvement from time to time. Don’t cheat and go over 120 for any appreciable time. As few seconds is ok but not a few minutes. A performance test like this will be much more informative.
Another depressing note about aging to consider: I have noticed that the older I get the harder I have to paddle just to stay in the same place. Improvements in fitness and performance come much slower and require much more work than they did 30 years ago. The take away for older athletes is: DON’T STOP.
I hope this helps.