I do prefer the First Five year mission...
Except Kirk in ST:TMP seems to be saying that even one five-year mission is a big deal that makes him uniquely
qualified to deal with V'Ger.
Quite possibly there never was another five-year mission in the history of Starfleet, much less in the history of the Enterprise
. At least such things are never referred to in the aired episodes and movies, and the very concept of an X-year mission really only exists in the opening credits of TOS. For all we know, the opening credits speech is something Kirk dictated for his memoirs, decades after the mission, with the unique knowledge that the mission would involve him for five years; all starships would have missions of indeterminate length, and Kirk calling this one a five-yearer is just marketing for the memoirs.