There are way more problems then just breathing and heart rate.
1. to heavy per height ratio, this would cause severe joint pain after the 20k mark
2. way to much muscle, again the more muscle the more prone to soreness and lactic acid.
3. Dehydration, even if they drink consistently throughout the race their electrolytes in their large muscle groups would be depleted in no time due to too much muscles.
4. Glycogen depletion as well, all that muscle will use up the stores so fast and soreness will creep in, they would not be able to walk towards the end.
and of course there conditioning in terms of lungs, however I think this is the least of their worries.
So I suppose it's possible (would take a miracle) but not probable and extremely unlikely unless they dropped 50lb of muscle first.
