As others have said, start small. That's good advice. You won't realize how badly untrained hands and arms can feel after a relatively short ride (not to mention good saddles your butt might not be accustomed to...).
The next bit is comfortability on the bike itself -- and this really means understanding what you like, not what The Right Formula has to say about your body. I've ridden a BSO (seriously, $120, fully-rigid MTB type, with my own $30 saddle) over 8,000 miles and loved it, and ridden a $6,000 "dream bike" less than 200km before deciding it was destined for the trash. I would have preferred to tour on the BSO MTB than the expensive bike. But this will usually not be the case. In any circumstance, though, you should ride whatever you intend to tour on several thousand miles before setting out, unless you're rich and can afford to replace gear until something happens to fit just right.
(I destroyed the BSO frame in a spectacular crash where it was ridden, literally, into the ground and I wound up unhurt -- whew!; and the amazing $6,000 pile of technology was traded to a rich guy who likes bikes,)
Once you get used to commuting distances (I mean > 40km a day or so, and this implies you've already done the climb through 5km/day to 40km day -- which is its own adventure, but one which is easily researched) in your own town then you should start riding to unfamiliar towns to find out what it feels like to ride every day somewhere you don't know. This matters a lot more than you might think at first,
The last bit, and the part I think that is most often overlooked, is the idea of learning how to become comfortable living in the open when necessary. I could probably ride from Fukuoka to Sapporo sleeping in a family hotel every night without any issue (Japan is well settled), but I would be hard pressed to do so in the US or Canada (where inhabited regions can be separated by vast spans of literal wilderness). And that brings up another point: self defense. I'm not going to get into this here, as quite a few cyclists hold extreme emotional investments in one side or the other of the concept of violence, but it is something you should contemplate if you plan on actually circumventing the landspace of the globe on a bicycle. Along with self defense comes the other issues of living alone in the open: you need practice.
Camping, cooking, cleaning, staying healthy, not getting depressed (which, when alone, really boils down to genuinely understanding who you are), not getting manic, getting enough sleep, not falling into a state of unmanaged sloth, not feeling like crap every day, not carrying too much weight, not carrying too little gear, carrying too much cash, not having access to enough cash, etc. are little things that will really bite you once your body is physically adjusted.
Anyway, have fun. Take everything everyone here has to say with a grain of salt (especially if they are touting particular gear or price ranges). You're the only one who will know when you're ready.