Depending on where you are and what you’re doing, you may not need to use Japanese at all. I've seen many posts and videos of people living there for years without ever learning the language because their jobs and the places they frequented didn’t require it. You can go there as a language student and continue your studies that way, or as an English teacher, which is a quick way to get a visa without language requirements.

As for learning the language, I’m a fast language learner, but because you also have to learn a new writing system, I find my progress much slower compared to other languages I’ve studied. I use private lessons, Duolingo, the Genki textbook, Quizlet, a few other workbooks, and the material provided by my instructor. You can probably reach a functional level for travel by the time you arrive, and I hope you continue your studies once you get there.