I bet that mother just had no idea how short a mile is. The US has even less of a pedestrian culture than a bike culture in some areas. I am often flummoxed by people who are impressed that I walk a mile to work every day; some of these people go to the gym three times a week and can certainly walk a lot further than a mile. They just don't know how long a mile is, not in walking terms.
They are the same people who are floored by my boyfriend's two and a half mile bike commute, when of course the reality is that his entire commute time is spent at traffic lights, since the distance is nothing at all.