Pioneers! O Pioneers!
Those long lines you see in the second and third lines of each stanza in this poem are pretty much Whitman's trademark move. He tends to write long, overflowing lines. This was pretty groundbreaking at the time, and that he would want to do something groundbreaking shouldn't surprise us after this poem. After all, he does love the idea of pioneering new ground. Even his lines have to push past the borders of the page to seek new, uncharted territory.