What Puredoxyk says is true: a person's conditioning has little to do with whether they get altitude sickness or not. In my experience (three summers at
Philmont, asst lower 48 peaks), the
brawny guys actually come out worse because of a "tough-it out" ethic, which leads to more altitude->worse symptoms->more distance to retreat.
Decidedly unbrawny, my unscientific pre-trek plans involve smoking, which gets the body making additional oxygen carrying red blood cells.
As this process worsens, you have leakage of water from cells (normally atmospheric pressure keeps tensions in balance). This leads to fluid in the lungs and swelling of the brain. It is then time to break out the diuretics, steroids, and Gamow bag.