Congratulations on booking your first cruise! You booked a great one. Alaska is one of those places where it's easier and, many times, more efficient to go by ship. It's certainly more beautiful than any other way.
You'll have a great cruise on the Century, with good food, service, entertainment and ambiance. There's usually an Alaska specialist onboard, who will get on the Public Address system when the ship is sailing, to give everyone standing on the outdoor decks some information about glaciers, the geology of Alaska, history, etc.
I don't think you'll be disappointed in either the ship or the destination.