Keats uses the extended metaphor of exploration to describe that wonderful moment when he first delved into Chapman's translation of Homer. For Keats, reading Chapman's Homer was as much a voyage of discovery as the expeditions of Spanish explorers like Cortez. Much the same could be said of Chapman's original translation process. In the field of translation he too was a pioneer, traversing hitherto strange, unconquered lands. He was the first man to attempt the arduous task of translating Homeric rhythms into English, and Keats's poem, with its extended metaphor of exploration, pays him due homage.
Keats's extended metaphor is also a tribute to the scientific method and its extraordinary discoveries, both terrestrial and astronomical. He makes specific reference to Cortez's sighting of the Pacific Ocean (although it was actually Balboa who made the discovery, but then his name wouldn't have scanned). He also refers to "the watcher of the skies." This relates to William Herschel, the British astronomer who discovered Uranus in 1781.
If you will refer to the sonnet you will see that the extended metaphor begins with the very first line. Keats is not talking about his travels or about foreign states and kingdoms. He is using an extended metaphor to describe his experiences with reading. Traveling in the realms of gold is experiencing all sorts of things in his imagination by losing himself in good books. The metaphor continues all the way to the line in which he says, "Yet did I never breathe its pure serene / Till I heard Chapman speak out loud and bold."
That is the end of the metaphor. The rest of the sonnet is an extended simile, because Keats uses the word "like" and goes on to compare his feelings upon first looking into Chapman's translation of Homer to those of an astronomer discovering a new planet or like Cortez when he first saw the Pacific Ocean. So the entire sonnet is one long metaphor followed by one long simile.