How is Wordsworth's "The Prelude" an epic poem?
The Prelude was intended to be a preface to another epic poem called The Recluse. However, Wordsworth became more involved with The Prelude and therefore never finished his proposed epic The Recluse.
Although The Prelude is a preface to a proposed epic, it does have some epic elements. Whereas other epics involve some type of journey or adventure, Wordsworth's poem involves a journey into his own imagination. Wordsworth searches his past life in order to find evidence that supports his inclination to be a poet. In tracing his life, Wordsworth uses the epic trope of describing a journey. Dante's Inferno and Homer's The Odyssey both describe journeys. In Wordsworth's case, his adventure is revisiting the journey of his life. This is fitting considering that Wordsworth tended to recall the experiences of his youth in an effort to revitalize his poetic inspiration.
The Prelude is divided into books and this is also characteristic of epic poetry. Like other epics, this poem is told in grand style. It suggests that the subject matter is of great importance. We might call Wordsworth the epic hero, although it might be more accurate to say "poetic hero." In either case, Wordsworth is the one who makes an introspective journey.
Even though Dante's work seems to be about a literal journey into Hell, it is also a journey of self-discovery. Such is the nature of Wordsworth's poem as well. Note how Dante's poem starts:
Halfway along our journey to life’s endI found myself astray in a dark wood,Since the right way was nowhere to be found.How hard a thing it is to express the horrorOf that wild wood, so difficult, so dense! (Inferno)