It's also price, which varies with distance more than with time.

And also the number of alternatives.

I might choose to allow an hour to get to Euston from some places in
zone 2, as it happens, but it would also take me two hours or less to
walk there from most of zone 2 if absolutely everything went wrong.

Oh yes. In my Cambridge commuting days, when everything worked I was
pretty smug, but when (for example) the train sat down at Letchworth I
didn't have the option of hopping on a bus with my Oyster. To say
nothing of the numerous times when the portion from Kings Lynn refused
to mate with the portion sitting at the platform in Cambridge, often
leading to the whole thing being cancelled (with me a good 50 miles
from my desk...)