Both 'farther' and 'further' are acceptable adverbs in this example.
When we describe distance, both farther and further can be used. These adverbs become interchangeable in this case.
- 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.
Further (not farther) can also mean 'additional', 'extra', 'furthermore', and 'more advanced'.
Do not use farther to describe theses meanings.
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NOTE: If in doubt, just use further because it encompasses both meanings.