Also being "fit to run" is more than just body weight. The vast majority of people can't run more than 2-3 days in a row without getting sore knees, hips, or ankles. A 30min run is about it for the vast majority of exercisers.
I'm in the Army. There was a time back 10-20 years ago when physical training was running everyday, push-ups and sit-ups. The rate of stress fractures and other injuries were astronomical. The Army changed to less running, only running every other day, and muscle failure/muscle strength on the other days. Injuries are less while the 2mi run times are the same.
What I'm saying is even for guys who workout regularly running more than 30min every other day is going to greatly increase injury risk.
So part of being "fit to run" is being fit enough or genetically gifted enough to run multiple days in a row. As an example: my very first week of organized running I ever did was a 40mi week. If you have 100 people who have never run before and have them run 5-6 miles a day for a week 90% or more wouldn't make it to the end of the week without an injury. If all a person does is run for weight loss it's not a very efficient modality.
Also the article is stupid. Anyone, given enough motivation anyone can cover 26.2 miles on their feet and be considered a "marathoner". Anyone who actually runs the entire way is likely going at least 9-10:00 miles. If you're running at least 9-10:00 miles you're probably not obese.
Alan