You have to be able to ride a loaded bike for the distance you plan to ride for the number of days you plan to tour. It isn't a race, you get no award for arriving early nor are you penalized for being late, there are no style points, and you can always stop and rest or walk your bike.
To me, the issue isn't distance, as you can adjust that day by day. It is getting accustomed to riding a bike laden with all your gear. Bikes handle differently when they are loaded down. A comfortable bike can turn into a torture device if loaded improperly or with too much weight.
Also, you should plan rest days into your tour schedule. I plan 1 rest day for every 4 riding ones.
If you are planning a tour doing X miles a day carrying Y amount of weight, then your training needs to insure that you can ride X miles carrying Y amount of weight. An even better idea is to make sure you can ride X miles carrying Y amount of weight two days in a row, which is the approach I use.
While some people prefer to train very little and use the first part of the tour as training for the rest of it, this won't work for "short" (less than 2 weeks) tours and, I believe, leads to lots of extra pain and mental anguish at the start. Personally, I don't recommend this approach.
Be warned that once you start bike touring, you may not be able to stop!
Ray