This is a great question. Certainly, I don't doubt the physical toll that the journey and the entire notion of living took on those migrants in the work. Steinbeck brings this physicality out in the description of the trials, the land, and the natural conditions that made life so difficult. Yet, I think that it is the spiritual condition that Steinbeck highlights in such stark detail. The primary theme of the redemption in hope and its sustenance through the darkest of hours is of vital importance to the novel. Consider Ma's words to this point:
Use’ ta be the fambly was fust. It ain’t so now. It’s anybody. Worse off we get, the more we got to do.
This idea of "the more we got to do" is a great statement of how the spiritual challenge to sustain hope and the connection to other human beings is the toughest element in the most physical of demanding conditions. There is physical difficulty, but it is this need to prevent acquiescing to the most base of conditions. It is the need to see things as they should be, as opposed to what is. These are spiritually challenging elements and the fact that the family continues to have hope and continues to demonstrate it becomes a testament to how spiritual faith is the base of all else. The physical challenges become cradled in how one demonstrates a spiritual connection to such elements.