For example, if someone inactive and obese decided out of the blue to run five miles every day, I would expect it to end badly.

It is nearly impossible for a low-willpower (obese and inactive) person to suddenly get so much willpower. He will be panting at 300m, feel his legs are made of lead at 500 and the rest would be sheer will? Nope. Okay there are always outliers and some drug users, but generally, no.