It is also possible you were over-training in your "moderate" intensity or conversely not increasing the load/distance enough. Training over years should result in good improvement even without HIIT. You will need to put a lot more time in.
To be clear, I was objectively in good cardiovascular condition before adding HIIT. My resting heart rate has been in the 40s for years. The surprise was that there was so much easy margin left by adding HIIT to my regimen.
I’m more interested in getting into the 90+ percentile range on the cardio metrics where I’m closer to the average. I don’t obsess over it, it is just interesting to see how different activities that I was going to do anyway move the needle.
My average heart rate on the test was 58, lowest was 21 (for 3 seconds), and highest was 250 (for 6 beats). A few minutes after getting in bed, my heart rate regularly is in the 40s and sometimes low 30s.