The ground seems to have gotten harder anyway. In adolescence, I can recall the occasional run across a gravel parking area w/o wincing. New NB 990s used to feel like on an air cushion (as well as earlier models). Some of it is probably loss of subQ tissue. Just being tired of sore feet plays a role. I had that driven home at a Blue Hills traverse.
I ran my first marathon (~1978) in a pair of $3 Adidas with zero cushion. No wonder my knees were sore afterwards.
Recalls a marathon in bare feet. I found this You Tube
video showing Abebe Bikila winning the 1960 Rome Olympics. Started at sunset due to heat.