There are many themes in this story. One has to do with duty. Phoenix Jackson has traveled the path many times on the same mission--it is her duty to do so, no matter how far or unpleasant it may be.
Race and prejudice are also prevalent themes. Phoenix faces the farmer who threatens to shoot her, and other instances of disregard simply because she is female, old, and black.
Another possible theme is resurrection. Phoenix's name alone refers to the mythical bird, the Phoenix, who wears itself down to nothing looking very ragged and old then dies by spontaneous combustion only to be reborn again from the ashes. There are many references to Phoenix Jackson being like this mythical creature.
As far as diction goes, Phoenix speaks in the manner common to her region and social status. She leaves out words, runs them together, and in some cases this not only represents her social position and education level, but also her level of fatigue in the story.
Hope this helps!