It’s the blood pumping through your arteries. I used to get this even in my teens after very long walks. You’re literally just feeling your own pulse as your heart works a bit harder to meet your body’s elevated demand for blood/oxygen.
I’m not sure why it seems to be more apparent after milder activity, but maybe something like walking doesn’t dialate your blood vessels so much so there’s a bit more pressure at certain points?
If you want to confirm its your blood vessels and not muscles, check your pulse as it’s happening and see if its the same rythmn.
As far as I know it’s normal, since I was in peak physical condition at that age (a lot of athletics, running etc.) but if it seems unusual for you personally I guess bring it up to a doctor.