Neither the athletes nor the nerds will be at a more advantageous position at the start of a catastrophe. However, as time goes by, the atheletes will be more likely to survive the following conditions: they can withstand lack of food and water better, they are able to travel faster and farther. Natural selection will only start to take hold on the following decades and centuries, once the actual direct effects of the cataclysm begin to fade out.
By that point it will not be about individuals; social groups, communities and other groupings will out-survive any single individual quite easily. That is the reason humans developed into social animals in the first place: there is great strenght in groups, even if any single member of the group is not physically or mentally equal to an opposing individual.
So, sorry to say to all nerds out there, but your survival during the first few days is pretty much arbitary, just as it is for anyone else. But should you survive the first week or so, personal physical fitness will begin to play a key role. If you survive the first few months, then your chances go up with every passing week; by that point you will have began to develop a liking for both quick sprints and long-distance walking endurance.