* = FYI, if you are not familiar your anaerobic threshold is 80-95% of your max heart rate and can be calculated this way: (Your age - 220) x 0.80 = Minimum Anaerobic HR.
Let's turn that around. The rule-of-thumb formula is actually (220 - age) X .80
and some formulas use .85
The formula is approximate. Obviously there is individual variation because of genetics, endurance, fitness, over-heating, nutrition and more.
So for me, age 70, it is (220 -70) X .80 = 120 , or if using .85 it is 127.
This means that a heart rate of 120-127 is a sustainable
rate all day long. I can, and do, but at a necessarily slower pace than 20 years ago
For those not counting pulse or using pulse watch, clinical clues to that rate may include varying amounts of pounding in your head, needing forced respirations, or breathless while talking
People can go above their anaerobic threshold for a while, then use a "cool down" period to pay back the oxygen debt, as in interval training.