Two things: get fast, and get proficient at distance.
Speed gives you time and options on big mile days. Even if you don't use it frequently, it's good to have from time to time. HIT will build speed. Pick a HIT plan, stick to it.
A lot of good can come from long days in the saddle - working out comfort issues, learning how and what to eat, learning to stay hydrated, dealing with issues that will arise, learning how to be efficient off the bike. A monthly 200k, if you can start at that distance. If not, start at 100k and work your way up.
Eventually you're going to want to get some back to back long days, to learn what that's like. Before that, you want to have all the things above pretty sorted out.
Having said that, what you're describing is a pretty big challenge, even for a veteran rando. Physically, logistically, and emotionally. It's also going to be somewhat expensive. I'm not going to say impossible, because that's not true and I don't know what you're capable of. Most likely you don't either.
I think six months is probably unreachable; a year might be enough, if you commit pretty deeply and make preparation your primary focus for the next year. Doing something like this involves getting close to what you're make of, testing your tenacity, or whatever cliché that gets the idea across: this is really quite hard and quitting is always right there. That itself can be an engaging journey.