"Farther" is used literally to describe matters of measurable distance (I think of the imbedded word "far," suggesting distance); "further" is more figurative and is used for broader general comparison:
Long Island is farther away from Cape Charles than Cape May.
Antarctica must be explored further.
She is further along in her schooling than I.
For more on this particular usage challenge, visit this site:
"Farther vs Further [1]" at grammarlyblog