Much have I travell'd in the realms of gold (1)
Keats tells us from the beginning that he is voraciously striving to be a great poet by giving us a metaphor of his deep reading. It's not just bragging—he is searching for something.
Till I heard Chapman speak out loud and bold (8)
Even at a young age, Keats knew he wanted to do something great. He pursued several careers before landing on poetry, because he believed that was the most pure and powerful way to reach people. He pictures Chapman here as literally speaking out to him, even though he's been dead hundreds of years.
Then felt I like some watcher of the skies (9)
Note that Keats isn't just portraying himself as a better poet having read Chapman. Here he's portraying himself more literally as a visionary, someone who looks beyond his surroundings for a glimpse of the bigger picture.