Farther vs. Further

Both 'farther' and 'further' are acceptable adverbs in this example.

Distance -
When we describe distance, both farther and further can be used. These adverbs become interchangeable in this case.
For Example: 
- How much farther do I have left in the marathon?
- How much further do I have left in the marathon?

In both instances, the meaning is the same.


Additional -
Further (not farther) can also mean 'additional', 'extra', 'furthermore', and 'more advanced'.
Do not use farther to describe theses meanings.
For Example:
- For further information, please comment on this blog post.

NOTE: If in doubt, just use further because it encompasses both meanings.

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