Kirk's "five year mission" was no different than the Cylons' Plan on nuBSG - that is, Roddenberry wanted to put together a dramatic monologue for the opening credits so he made up something that sounded grand. You can see the memos back and forth between him and Justman as they add and subtract stuff from the speech. In fact, the format of the show as it was pitched and explained to people working on it was that Kirk's job was like that of a colonial naval commander from the previous century - support outposts, chart new seas and territories, explore, serve as the representative of his government to lands so distant that timely communication was impossible. So on and so forth. Oh - and defend the realm.