I think you need to have enough miles on your bike to know it's working for you. The rest will come. As long as you are flexible in your stopping times, you stop when you want to, and you will go further over time. I haven't done extensive touring, just occasional weekend/overnight trips, but I find that in spite of a lack of long-distance days under my belt, I can get where I have to go by pacing myself. As the day wears on, the breaks may get more frequent, but that doesn't bother me as long as I'm on target to reach a place I can sleep for the night.

My first real trip of any length was bike-camping to college, a couple hundred miles in 3 or 4 days. I wish I could remember the exact route, but I do believe that prior to that trip 30 miles was about my max daily ride, but on that trip I did double that daily mileage while carrying far more gear than ever before. I know that I walked up hills that were steeper than what I was used to, and I know that breaks got more frequent as the day wore on, but I don't think I ever failed to make my scheduled stop by sundown. Breaks usually involved a couple of minutes to sit, drink water, and stare at the map every hour or two plus a half hour or so at lunch.

I feel like, apart from actually riding your bike, training is only necessary if you have solid mileage requirements and/or you have to keep pace with someone else. On a long enough tour, you will acclimate. On a short enough tour, you will may never have time to really adjust, but a rough day of touring still beats a day "training" to me. I like to have somewhere to go, so biking just to prepare for biking later isn't very motivating. But keeping an eye out for opportunities a little outside of your comfort zone can create excuses to get miles in without it feeling like training and it also gives you an idea of how far you can go at a stretch.