Most people in somewhat reasonable shape could "run" a marathon on zero training. They might take 7 hours and have some minor injuries by the end, but they could finish.
To add a personal anecdote, I once walked 25 miles in about 8-9 hours* with a heavy bag, and then showered & walked to work afterwards. I was kind of out of it, but definitely not falling-down-tired or anything like that.
*I severely misjudged the length of the trip before setting out, thought it was about 5-6 miles.
*I severely misjudged the length of the trip before setting out, thought it was about 5-6 miles.