What, according to Krakauer, convinced Chris that he cold live in the "meager rations" he brought to Alaska?

From the text:

On January 16, McCandless left the stubby metal boat on a hummock of dune grass southeast of El Golfo de Santa Clara and started walking north up the deserted beach. He had not seen or talked to another soul in thirty-six days. For that entire period he subsisted on nothing but five pounds of rice and what marine life he could pull from the sea, an experience that would later convince him he could survive on similarly meager rations in the Alaska bush.