Just to be clear, it is not the case I encourage all 6 year olds to run Marathons. This example, however, is comparable to the effort put in by this kid - 8.5 hrs.
In our travels, we enjoyed walking clear across the town we visited. On the way, we would stop at playgrounds, indulge in fancy cake/drinks and refresh with organic fruit from the open air markets. In all, we easily covered 25 miles over the course of a day. I remember specifically our round trip from the Forum Beach to Camp Nou in Barcelona. My youngest was 8 or 9 at the time. Parents walked at a steady pace but the kids would be running back and forth nonstop pointing out things they wanted to see or snack on. At the end of the day, the parents were beat. The kids seemed to have plenty more energy. Eventually, the kids fell asleep, suddenly, the moment they sat in bed to watch TV.
I remember doing similar things with my parents. I also remember summer play when, between the various outdoor games, pickup soccer games, going for a snack in the neighboring shopping area, etc, I would clock 20-30miles per day. There is no physical ailment I have in my middle age as a result of running all day long as a child. And, let me make this clear, the implication from CoachY that this 6yo necessarily should love running later in life is flawed. Maybe he will pick something like volleyball, table tennis, soccer or basketball, just like I did. I may have picked running but organized running was not an option whereas the others were. But I never shied away from walking long distances.
Given my experience, I'd say, let the kid run if he wants to run. If that 6yo complained that he wanted to stop, I am sure someone would have intervened. Running does not need to be in his future, the memory that he completed a marathon when he was 6 inevitably will. I walked clear across Milan when I was 10. My parents had dropped me off for soccer practice across the city and the practice was cancelled but cancellation was not properly communicated. Someone loaned me a bike but then I got a flat tire and had to push the bike most of the way back home. An ice cream vendor saw me a little pooped out and handed me a soft serve. Got home really happy and full of energy. I would hate that memory to be taken away from me by some of today's eccentric police. I would argue that because of that roaming I did, I was perhaps healthier than many of my peers who grew up in more protective settings. My immediate friends did exactly the same things I did at that age. When my own kids hear of my stories, they begin to hate living their childhoods in this age, today.
