It is a pleasure to ride a quality bicycle that fits one's body. A bike that is too small, has worn-out components, is not set up properly, or is otherwise not quite right, might be fine for short local rides. But it won't any fun when you spend four, six, or more hours per day in the saddle.
If you use an ill-fitted or uncomfortable bike for "training," you might gain a little in the way of cardio-vascular conditioning, but not much else.
To best prepare for bike touring, get the right bike, and train on it.
(I have two well-fitted touring bicycles, and I use them for everything.)
By the way, not everybody is inclined to train before a trip. Maybe I should, but I usually don't!