You don't HAVE to train. Due to a hectic life schedule, I frequently go months without ANY riding then will "train" for 100-200 miles total before heading out on a month+ long tour. I do short days (20-35 miles) at first then add in 5 mile increments if possible. I'm mid-50s so yes it is a little tiring the first week but after day7-10, I am in good enough shape to ride in the Rockies. I also have very low gears to be gentle on the knees. Of course, I am not a speed demon at first but I get back to a 12mph average by week 2.
Yes, it is much better to train before the ride as much as possible but you don't have to. You just have to choose whether you would rather tour even though out of shape or not tour at all. For me it is the former.