For starters, read this:http://www.adventurecycling.org/features/getinshape.cfm
If you have sufficient extra time for the trip (perhaps a few weeks longer than otherwise), you can train en route. (This won't work, of course, if your riding with a group.) So that would mean that you don't need any real "training", at least not of your muscles. Without such training, however, you must expect a certain level of discomfort--of your leg muscles, your butt, your arms, your lower back and your neck. These are all things you can work through with sufficient time, either before the ride or duriing it.
You should, however, ride your bike sufficiently to make sure that it fits well and does not aggrevate any of your joints, that it has the right gearing, and that your butt and your saddle are well suited for each other. You should also ride enough with your equipment to make sure that it is all well suited for training.
You might also want to ride enough to learn how to handle a wide variety of conditions, such as cracks in the road, crossing railroad tracks, minimizing the risk of being hit by a car from behind, the side or the front, riding in the rain, handling slick conditions, handling loose dirt and rocks, looking back without swerving, standing while pedaling. These skills can be learned fairly quickly, but it takes a bit longer to make them second nature.
You also want to spend enough time learning how your bike works and how to repair simple problems. You certainly want to know how to deal with a cut tire, flat tube, broken spoke, dropped chain, broken chain, broken cable, cable out of adjustment, derailleur adjustment, etc. And you want to know regular maintenace such as cleaning and lubing your chain, replacing your chain, replacing a tire, fixing a flat and replacing a broken spoke.