You know what they say about the candle that burns twice as bright, though
If there any evidence that intense mental loads somehow wear down the brain, or the body as a whole, any faster? If anything, I only know that intense mental loads stave off the onset of Alzheimer's.
At the same time runners tend to be the kind of people that are very concerned about their health, so they eat healthy food, with moderation.
So overall caloric intake of a runner can be much less than that of obese couch potato.
Compared to what though? The average sugar-addicted citizen? Someone who does no exercise?