The Prelude is Wordsworth's account of his artistic development, whatever it was that made him a poet. Books, for example (Book Five), and key episodes from his childhood, his early schooling. Not - definitely not - college. Wordsworth presents himself as a Cambridge goof-off, "an idler among academic bowers" (1850, 8, 503).
The bulk of The Prelude, though, is a form of travel writing. Journeys to Switzerland, France, and Scotland are central, and presented in detail. London, too, a bit surprising for someone so strongly identified as a nature poet - see the chaotic Bartholomew Fair scene at the end of Book Seven.
The last book begins with a trip to Wales and an night ascent of Mount Snowdon, with a friend and a shepherd guide:
It was a close, warm, breezeless summer night,
Wan, dull, and glaring, with a dripping fog
Low-hung and thick that covered all the sky;
But, undiscouraged, we began to climb
The mountain-side. (1850, 14, 11-15)
They climb and climb, and then:
For instantly a light upon the turf
Fell like a flash, and lo! as I looked up,
The Moon hung naked in a firmament
Of azure without cloud, and at my feet
Rested a silent sea of hoary mist.
A hundred hills their dusky backs upheaved
All over this still ocean; and beyond,
Far, far beyond, the solid vapours stretched,
In headlands, tongues, and promontory shapes,
Into the main Atlantic, that appeared
To dwindle, and give up his majesty,
Usurped upon far as the sight could reach. (1850, 14, 39-49)
The stars are visible, too, and the only sound is a distant waterfall. A sublime scene. What's the point? The young man discovers, not in an instantaneous vision but "in calm thought reflected," "the type of a majestic intellect... that feeds upon infinity, that broods over the dark abyss." This intellect is somehow simultaneously external (God or Nature) and internal to the poet (Imagination). This sublimity is a "power, which all acknowledge when thus moved" but only the poet "can send abroad kindred mutations; for temselves create a like existence."
So far, so good, I guess. This fits my own experience of nature, which helps. I don't know how to move to the next step, but it's not clear to me that Wordsworth does, either. Here I either lose the thread of the argument, particularly the later link to "spiritual Love," or fail to see the intuitive leap.
Wordsworth is clear enough about the value of poetry, of doing poetry. This imaginative communion with Nature is "the highest bliss that flesh can know" and "endless occupation for the Soul":
Oh! who is he that hath his whole life long
Preserved, enlarged, this freedom in himself?
For this alone is genuine liberty:
Where is the favoured being who hath held
That course unchecked, unerring, and untired,
In one perpetual progress smooth and bright?— (1850, 14, 130-5)
Wordsworth admits to "lapse and hesitating choice, / And backward wanderings along thorny ways," but that favored being is obviously him, the poet and genius. That latter word - perhaps that's the missing step.