Never trained for touring. I was a bike commuter, though, so wasn't desperately unfit.
The thing about touring is that you make up your own schedule. It isn't a race. If you start out unfit you go less far, less fast than you might towards the end when you have ridden yourself into better shape. So unless the first tour starts in seriously mountainous territory (in which case I'd recommend starting fit enough to at least ensure it's a cycling tour, rather than a walking-beside-a-bike tour) no training is required.
As for staying in shape to tour, it isn't an issue, both for the reasons given above and because I routinely spent around 10-12 hours per week on the bike.