What you are experiencing is a common and unavoidable result of aging. Max HR drops with age and there is no training or secret sauce that changes or even slows that down. This DOES indeed act as a governor because your cardiac output is the product of the heart’s stroke volume (which usually declines with age as well but we can slow that decline) and the heart rate. Your maximum cardiac output is the amount of oxygen-carrying blood your heart can provide to your muscles. Clearly when the the maximum HR you can reach drops the maximum cardiac output is also going to drop. At 70 I am in the same boat. In my 20s and 30s, I used to race for 3 hours with a HR of 185. Now it takes a massive effort to hit even 160.
This unavoidable decline in cardiac output is why Maya has you so focused on improving the metabolism of your muscles. They still respond as they did when you were young.
Have a great trip to Shasta.
Scott