Well, unless one is severely overweight, walking is a very light activity - it actively employs very few muscles and has a very minimal cardiovascular impact.

However, this is a bit odd in context. If one considers a population where 60%+ of the population has 30%+ of bodyfat percentage, then it's impossible to define what constitutes fitness - even one flight of stairs, in a sense, requires a decent amount of training.

In at least northern Europe, hiking is relatively common for elderlies (people in their 60s) - and this amounts to hours of walking in rough terrains, so definitely, for a person who's not (considerably) overweight, and hour of walks in the city is... a walk in the park.