From my experience it depends dramatically on age. In my twenties I would ride up the local mountain, about 2000 feet, once or twice, and then I was good to land in the Alps. Thirty years later we try to get in 6-8 rides with 5000+ft of climbing, in addition to that same 2000ft ride most every week of the year, and we don't really get into the groove on a tour until about the third week.
It also depends on the length of the tour. If you are going for months then it doesn't really matter. If you are going for weeks it is nice to be in pretty good shape at the start.
I can't do the trainer, I would rather ride in the snow. Too boring for me.
Touring for me is 12mph, all day long, day after day, definitely not a sprint.