Further vs Farther

I know this one, but while editing today, I had a brain freeze and had to refer to my cheat notes to choose which word was correct in my sentence:

The farther they travelled, the lower the ceiling became.

Was I right or wrong?

Farther and Further

The word farther implies a measurable distance.

I threw the ball ten feet farther than Ed.

Further should be reserved for abstract or hypothetical lengths.

The executive climbed further up the ladder of success.