How Did Wordsworth Arrive at the Choice of Subject for the Prelude?

In Paragraph 6 Wordsworth contemplates ideas for the poem. He sets off with a positive frame of mind preparing to write, "For such an arduous work, I through myself make rigorous inquisition, the report is often cheering" he seems positive about the challenge he has set himself. We follow the changes within Wordsworth's mind through the lists he includes and the thorough detail he applies "Will settle on some British theme, some old Romantic tale by Milton left unsung" this effect reflects his ideas flowing to the reader.
Wordsworth referred to the poet Milton, who had planned to write an epic poem known as Paradise Lost. …