The grammar police say:
When dealing with a progression of distance you use 'farthest' not 'furthest' - in all other cases it is 'furthest'.
So . . .
This was the farthest he had ever been from home, ending up in Northern Africa was the furthest thing from his mind when he set out on his bicycle that morning.
But to be honest the two words are so misused that they are pretty much interchangeable these days. Lesson over for today, but be on you're guard.
(Go on, it's sitting there.)