I walk fast cause I'm 2 m tall and have long legs, but my ex used to walk a Pokemon go route with me that was 1.6 miles and it took us just about an hour to go through it and back. She was not very tall and had no problem keeping up.

Around 5,5 km from me to my Grandpa on the other side of my city with pedestrian lights etc. Takes me around 45-50 minutes. Fast walker, around average height for an EU man.

Yeah my sustainable walking pace is around 4.5kph on flat terrain. And I'm a pretty quick walker. Which is around 2.5mph. 3 miles is probably a 70 minute walk for me.

15 minute mile is my "walking while listening to fast music" fast pace. It is definitely a brisk pace but this is not even close to jogging. And 3.5 miles is my normal distance.

And even then, the point is more that if you are moving at 4 mph in your car, the car is doing almost no work. 4mph for a semi in shape human is perfectly reasonable to walk, so if that is all your car is providing you, it isn't worth it.

As a 5' person, 3 mph is a sweaty pace for me even in cold weather. Someone with long legs might find 5 mph easy but I'd be running.

well yeah, most people these days are extremely unfit, that's kinda the point.

Walking fast isn't a superhuman feat, it's something you become able to do if you regularly walk a lot.