For another point of context, to my father, an Australian born in 1935, 6 km | 3.7 miles is the distance he walks every morning after he gets up at 4 am or so.
He's still pretty active, he maintains sections of the Bibbulmun Track (1,000 km walking track here in the lower corner of the state) shovels the odd few tonnes of manure, splits wood by hand, etc.