Everything of the following is perfect:
The use of dahin is not wrong, but only makes sense in a context with mentioned location, see below. Fahrt is used a bit more than Reise if the trip was in a car/bus or similar, but none of them is wrong:
(Car, etc.: generally Fahrt is the noun drive but also used with ship or balloon for example)
Es war eine 14-Stunden-Fahrt. or
Es war eine 14-stündige Fahrt.
(General: Reise is travel or journey)
Es war eine Reise von 14 Stunden. or
Die Reise [Fahrt/Der Flug] dauerte 14 Stunden.
(Plane: Flug is flight)
Es war ein 14-Stunden-Flug. or
Es war ein 14-stündiger Flug.
The use of dahin, merely a synonym to dorthin, is common only, if the location has been explicitly mentioned before- especially, if you want to emphasize the location, but isn't it the same with there?
Do you know to which city my boss forced me to drive? Istanbul!.
Then you can use:
Es war eine 14-stündige Fahrt dahin/dorthin.
or without that precondition:
Ich war in Istanbul. Es war eine 14-Stunden-Fahrt dahin !
Addition:
Others brought up da instead of dahin. But da as dort is only used, if you are already there :
Ich flog erst über London und hatte einen 14-Stunden Aufenthalt da.
Ich hatte eine 14-stündige Reise da.
(The last one is unusual but would mean that you started another trip there where you arrived :-)