It's crazy! We have a two year old and go to the zoo fairly often. Almost every other family has kids twice her age sitting in strollers. She can walk around for 4 hour stretches no problem. We were playing Pokémon Go when it was all the rage and she'd just walk along with us for miles at 18 months old. It makes sense that once kids get too big to hold for long periods of time they'd be able to follow along with a roaming band thousands of years ago (not rushing everywhere, but walking at a decent pace). Kids have the capacity to do a lot more than we give them credit for, and not allowing your kid the opportunity to walk for long stretches does them a disservice in the long term.