The opposite of far is farther, and the opposite of farthest is farthest. The phrase "further away" is improper and should not be used. Also, the words farther and further should not be used interchangeably.
The two words are not interchangeable. The term further refers to the physical distance between two points. For example, the map indicates that we must walk further. Also, the term is used metaphorically. As an example: This subject should be discussed further.