You don't need to train at all as long as A) you don't travel any faster than you are comfortable (which will get to a very credible performance within the first half week if you were previously out of shape but healthy); B) your group will wait for you, or better still you are currently the fastest or only member; C) Don't undertake a route that starts out in really nasty climbs etc.... That may seem too simple or something, but it's the basic truth, the main issues are competititve. For me I am just as happy plowing along at 2/3rd the normal mileage for the first couple of days, vs. doing thousands of meaningless miles in the saddle be fore I leave. But others enjoy the training as much as the touring.
If you train anyway, I would recomend you do it only on a loaded bike, and at realistic speeds. Work a lot on position, technique, and handling. If You are out of shape you need to go slow to give yourself a chance to build up your knees.