I don't like to use the word training. To me it implies a focus on the physical preparation of cycling.
A bike tour, in my world, is about far more than the cycling. Yes, we need to be able to cycle to bike tour, but it's not the only ability we need to enjoy it.
I prefer to use the term practising because it covers more than just the cycling.
For example, there's navigation, packing, cycling on different surfaces in different conditions, clothing for different conditions, eating & drinking.
If camping there's a whole load of other factors - finding suitable pitches, pitching in different weather, staying warm/dry/cool depending on conditions. Even being able to pass long cold nights is something that some people can't do comfortably.
If cooking, there's (again) cooking in different conditions, fuel considerations (especially in extreme conditions) as well as resupply.
There's also the psychological aspect; Having the confidence to listen to your body requires experience. Adjusting plans on the fly gets better with practise. Learning how to deal with hitting a psychological wall requires practise.
The more you practise these the better you will get, the more you'll enjoy it.
For me, I have a "Touring head", a state of mind, that gives me a real kick - otherwise I wouldn't tour. To me, being able to don my "Touring head" is more important than being able to cycle x miles with y kilos.
To be practical, practise when & where you can. Pick somewhere, load up the bike, cycle there, fire up a stove, cook, eat, relax, cycle back a different way. Even just camping out in the back garden in foul weather can be a great learning experience
Over the years I've met many touring cyclists. Of the ones that were not happy, not enjoying themselves, riding the bike was not the issue.
Good luck!