If this were Milton...

...there would be epic simile where that rock is:

She was an elfin pinnace; lustily
I dipped my oars into the silent lake,
And, as I rose upon the stroke, my boat
Went heaving through the water like a swan--
When, from behind that craggy steep till then
The horizon's bound, a huge peak, black and huge,
As if with voluntary power instinct
Upreared its head. I struck, and struck again,
And, growing still in stature, the huge cliff
Rose up between me and the stars, and still
With measured motion, like a living thing
Strode after me.
-Wordsworth, The Prelude (1805), I.401-412

In other words, that "when" in line 405 would function differently, so as to elaborate something about how swans are when they are doing so and so. And we'd have no dash (or semicolon, in the 1850 version), but instead that wonderful Miltonic enjambment that can introduce thirty lines or so (sometimes more with a sublimely equivocal "or") extending the comparison. What's important, though, is that Wordsworth--quite perversely, really, as with everything in the epic, lyric preface to a poem that is The Prelude--indeed gives us all we would need for that Miltonic simile. All the complex qualifications by heaping up imagery are there. It's like he's gutted the Miltonic form, partly by shooting it through with continual touches of actual or anti-metaphoric action ("I struck and struck again"), and partly (or mostly) because all these aspects apply to the rock. That's what's so striking about "like a living thing": the ghost of the simile insists on some level that we should still be talking about the boat being like a swan when ...it steals its way over water with measured motion, say, or when, from behind a craggy steep it glides, mounting the air, sailing smoothly aloft... And--in the final twist--Wordsworth is indeed doing this, since the measured motion is of course only the movement of his own rowing. But meanwhile the "living thing," has seemed both too obvious and too strange--or too strange precisely because, applied to the swan, it would have been too obvious. In other words, we shuttle between two metaphoric possibilities, one concrete and the other abstract: the first is due to the new tenor (the mountain), the other was called up not by the other tenor (boat) but by its huge and mighty form or gutted vehicle which should have been there (like an unbesotted, unspotted spot of time, like the impersonal form of time itself, like a "when"). And shuttling between them, moving through the concrete simile to the ghost of the other simile and its thwarted epic openness, is what causes the "living thing" to apply only strangely to the cliff. This is classic Wordsworth: what seems easy, even cheesy at first is actually doing a huge amount of work. More than that, it is allowing you to sink into the bits you naturally think are deeper: I doubt no one who reads this famous passage has any problem with "strode," however much they may balk at the commonplace "like a swan" and the almost banal (or at least horribly vague) "like a living thing." And I'm sure while reading it, they hit that inversion hard, and give themselves over to its wonderful lengthened "o"-sound, which lets them draw out the action of the cliff and give it that dark vitality. But these prosodic effects, which we might be quicker to accept as significant, are only substantial because of that subtle, almost hidden work those dismissed similes are doing. In fact, the passage is only read thoroughly and heard fully when, carried along by the rhythm and the first simile working together, we resist that temptation to pause and linger that the dash gives us at the beginning, and steal right over it, just as those who revel in the "strode" have to resist the temptation the spondee produces--for it turns out this was no mere inversion after all, but stress following stress--to move on after "me."