I believe trying to "ride into shape" on a tour is a waste of touring time, and a lot less enjoyable. Not being in riding shape does not provide reserve capacity when encountering an extremely hard day due to hills, headwinds or longer than anticipated mileage. As MassiveD indicated, pushing hard before your body is ready also increases the potential for injuries.
I enjoy riding, and do not find "training" for a tour onerous. Where we live it is hard to ride and not get a hill workout. My wife and I stay in good riding shape year around, so it is only a matter of tweaking the intensity and hill work. A few years ago on a loaded tour, we averaged a little over 50 miles/day for 74 consecutive days. While not mega-miles by any means, I felt pretty good about it for a guy in his mid 60's. We took several short days, but did not need a rest day. However, we had a lot of base miles before we started.
Most training regimes designed to improve fitness or strength use some variation of hard days followed by recovery days: overload/fatigue- adaptation- overcompensation. It also takes time to get into shape, and is probably not possible on a short tour. One training program, just to ride a 6 day tour, uses a 10 week progressive training schedule (Cycling Past 50, Joe Fried). I've seen similar schedules for "youngsters" that still utilize the 10 week time frame.
Something that is harder than being in physical (bike muscles, and cardio) shape, and something we have less control over is acclimatization. We have had some pretty rough tours when we left the cool wet climate of Oregon and ended up riding in a country with temperatures hitting 43C or 110F. If you know you will experience a wide difference in climatic conditions, allowing time at the start of the tour to get used to it will prevent any heat issues. Or if you can do some riding on hot days before the tour, all the better. This summer we were on a tour across British Columbia where the the temperatures ranged from 28F to 108F. It is pretty hard to prepare for those conditions. However, I think being in good riding shape at the start made a lot of difference on those hot, hilly days.
IMO- Be in as good as shape as you can at the start of a tour, and keep "training" fun and enjoyable. After all, you probably would not be considering a bike tour, if you did not enjoy riding a bike
