In The Good Earth, how did they travel south even though they were so weak?

In The Good Earth, Wang Lung and O-lan still manage to pack up their family and travel south even though they are really weak.  The two realize that their land will not support them, so they feel that they have no choice but to look for a better opportunity in the southern region.  It really is their sheer determination that carries them through the difficult journey.  They do not see any other choice, so they make a point to go south for their survival.  This manner of determination is seen on other occasions in the novel:  when O-lan is in childbirth, when the family reach the south and have to beg for money to eat.  So the concept of inner strength is not new to the family, and therefore may be seen as their driving force.