You're 21. Quick recovery is your biggest asset when starting a tour in less than good shape.

Begin your tour with 3 moderate days, take a day off and follow with a day of limited effort (whether you ride longer or not keep the effort low). Ride another couple days, take another day off and within a week and a half from departure you'll have a pretty good idea how your body is responding to the new stresses. By the end of the second week you'll be amazed at your gains in fitness.

Ride as much as you can now but I don't see any reason to worry about a serious training plan. Suggestions to do a few rides loaded seem wise in that you'll get a good feel for the bike and what the load feels like to your legs. Otherwise, your fitness will take care of itself in the first weeks of touring.