Farther is used to discuss physical distance. For example,
How much farther do we have to drive?
The farthest she has run at one time is 26.2 miles.
Further is used to discuss figurative or metaphorical distance. For example,
That couldn't be any further from the truth!
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Here's an easy way to remember the difference: the word farther contains the word far, which also related to physical distance.