That’s because I knew both were about distance, I just thought the right one was whichever sounded better in the sentence. Turns out there is a difference.
Farther is for physical distance. Further is for a metaphorical distance.
So if you can replace the answer with miles, feet, or centimeters then the right word is farther. For example, I’m farther away from Far, Far, Away than I was when I lived in Disneyland. And I need more coffee before I can get any further involved in this project.
Make sense now?