Final answer:
Krakauer's main concern as he began his descent was running out of oxygen.
Step-by-step explanation:
In the passage, Krakauer's main concern as he began his descent was running out of oxygen. As he ran blindly in fear, he hoped that by reaching camp, he would be saved by the boys. However, he also had a lingering thought that he might not make it to camp and could soon be stiff and dead.