This entry in Specimen Days shows how broad Whitman's youthful participation in literary culture was, including novels, plays, poetry, journalism, teaching, and even debate. He worked in offices in New York City and in country schoolhouses, which presumably was one source of his enthusiasm for the diverse work of everyday people in their various settings exhibited in Leaves of Grass. I imagine that his early experience with debate contributed to his confident tone as well.
My favorite part of the entry is when he parenthetically notes, "['boarding round' while teaching] I consider one of my best experiences and deepest lessons in human nature behind the scenes, and in the masses." I assumed this meant staying with students' families, based on a vague memory of hearing that this sort of thing happened back then, but I made a quick search to confirm this impression. A brief article from Michael Day on the Country School Association of America website describes "boarding round" as teachers living with their students' families, switching families every week or so: http://csaa.typepad.com/country_school_associatio/2007/04/index.html
As in Leaves of Grass, Whitman appears here to be someone greatly concerned with the goings-on of his fellow man rather than some aloof, reclusive poetic figure making grandiose statements from on high. In contrast, though, he certainly doesn't live up to his epic "Song of Myself" loafing reputation.