Sorry to be that guy, but further is typically used for figurative distances, while farther is preferred for literal distances. I always remember this because it was a plot point in a crappy movie with Sean Connery.
That wouldn't be The Avengers, would it? That movie was so bad that I've blocked out all memory of it, other than to remember how bad it was. It does tend to confuse people though when I tell them the worst movie I've ever seen was The Avengers.
In case it wasn't clear, I was NOT talking of the Marvel movie with the same name.
Thank you. I've been meaning to look that up for years.