Google "DOMS" - delayed onset muscle soreness. There's so much on it. It comes from how hard you're tearing muscles to build them up. Gross sounding, I know.

I bet somebody has even written about how to be the energizer bunny when hiking. Oh the blogs you will go to now!

Could be lots of things. Sometimes we get more pain-resilient when no longer being a sugar burner. Or something about how well hydrated you are before and after. Or maybe you're accidentally hitting a sweet spot, where you're working hard enough to get stronger, but not so hard as to overdo it. or maybe something about your rhythm of work and recovery means you magically provide rebuilding nutrients at the perfect time so there's no need for pain to remind you.

Would be fun to experiment and see if you can get sore, LOL or would that be weird. You have lots of time to play around as you learn about exercise and how your body responds.