I think that if you are a) reasonably fit, b) have no chronic medical issues, and c) have a bike that fits you, you can ride a ton of miles with very little training beforehand. Take it slow, stop when you need to, eat a lot and drink even more and you'll be fine. The main thing is pace. Don't try and push yourself too hard and you can do a good tour with little build up.

Distance from day to day is important here. If you feel fine one day and you do some massive distance, you might be feeling pretty rough the next day. Moderation is key.