This is an interesting question—and Wordsworth goes some way toward answering it for us in his title, which sets the scene for the reader. Note that he's composing this poem "on revising" Tintern Abbey—this is not the first time he's visited this place or observed these views. As such, the present view of Tintern Abbey which Wordsworth describes in this poem is colored by Wordsworth's history with the place and his memories of previous visits.

At the beginning of the poem, the speaker tells us that it has been "five years" since he has been here and that he has thought a lot about it during that period. In the second stanza, Wordsworth describes how "in hours of weariness," memories of this place have given him sustenance. He has dipped into memories of nature past, then, in order to achieve "restoration" in the present.

The speaker makes a deduction from this—he is assuming that "in this moment there is life and food / For future years." That is, because he knows how much he has relied upon his memories to improve his mood in the past, he is trying to soak up as much of this sublime natural scene as possible now, because he intends to use it, in turn, to restore him in the future.

So, there's a lot of thinking about time and the storing-up of nature for future recollection in this poem. The enjoyment Wordsworth derives from this pastoral scene is part of his past, present, and future.