You're normal. This is what happens, both due to training and aging, but in your case it's mostly training. Due to your increased training load, your aerobic system has become more efficient and your ejection fraction larger. Thus your HR is lower and your power is up, both. You're seeing what everyone wants to see. Try 5000 miles/year. That's only about 100 miles/week, what you're doing now, but year 'round. You'll see even greater gains. I find a reasonable max training load for me, where there's not a lot gain to be had from more hours, to be ~10 hours/week.
Like everyone on here told me, it's time to get a power meter. You can ebay a used hub, just be sure that it has the right number of holes and that it's an ANT+ wireless unit. Riders are moving away from hubs and to pedal or crank based systems, so hubs and complete wheels are available.
The PM gets rid of that uncertainly as physiological changes will keep happening. Until then, set up new zones off your new lactate threshold. No big deal.
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Results matter