I read the book. The ending of the book was ambiguous and has a different ending than the film, overall, the book was kind of a slog to read through. I do a lot of walking myself, about 4 miles on average a day, and in the book, if you fall below 4MPH you catch a warning. After three warnings, you get shot. Most of the time, I average 4MPH. Now doing that for 300 miles, day and night wouldn't be possible and I am in very fit condition. I'd probably catch it about mile 50 or 60.