You are forgetting a few things.
A) We don't know what "shape" the winner of the bet was in. Was he an athlete or morbidly obese? That's huge (no pun). If an obese guy wants to make such a bet, I'm more likely to accept. If he's a regular runner or last year's #1 Iron Man athlete, I'll decline
B) Fatigue is NOT linear. The guy could've had energy for 30 miles then started cramping at the next mile and until mile number 50. We don't know how fatigue built up for him, but we can assume that the marginal cost of the first 2.91 miles is NOT equal to the cost of the 20th segment
But yes, otherwise, your numbers look good, they just don't tell the whole story. Numbers almost NEVER do.