I guess I don't understand how someone can physically run a marathon and still be overweight. A girl I know just ran a marathon and I would never in a million years have pegged her as a marathoner. She ran around 4:30 which isn't fast but isn't terribly slow for a female.
Even the most basic training plans have you running at least 30 miles a week. It's like the rest of the time they must be eating crazy amounts and sleeping.