Don’t Say: You may be able to run further, but Mary can run faster.
Say Instead: You may be able to run farther, but Mary can run faster.
Here’s Why: Many speakers use these terms interchangeably, and you probably won’t get into trouble if you do, too. But conservative speakers argue that there’s a meaningful distinction between them. “Farther” refers to measurable distance.
“Further” refers to degree, quantity, time, or other qualities that are not being precisely measured. In this instance, we are talking about a measurable distance, how far one person can run compared to another, so “farther” is correct. Additional correct examples are:
Can you promise me there will be no further delays?
The map says the island is farther away than we thought.
We argued further about Terry after he had left.
He’s out of the wheelchair and walking a little farther each day.