Why was the July 10th hike a “tremendous mistake”?

After loading his few remaining possessions into a backpack, McCandless set out on July 10 to hike around Lake Mead. This, his journal acknowledges, turned out to be a “tremendous mistake... In extreme July temperatures becomes delirious.” Suffering from heat stroke, he managed to flag down some passing boaters, who gave him a lift to Callville Bay, a marina near the west end of the lake, where he stuck out his thumb and took to the road.