I'm now in my mid 60s. It has never occurred to me to "train" for tours. I ride enough in the ordinary course of events to stay reasonably fit, and touring is generally a low intensity activity, I'm not racing, so I go as far as I feel like or as is practicable in the terrain.
I'm not much of a planner. Experience tells me that about 100k/60m is a comfortable daily average for loaded touring, so I look at approximate routes with that in mind and adjust depending on the availability of accommodation, likely amount of climbing, all that stuff. And I'll improvise, if somewhere looks especially attractive I'll stick around, or I'll detour if I hear about something interesting in the vicinity. Even when not camping, I don't generally bother with booking accommodation ahead of time unless I'm going somewhere that's likely to be busy - for example, a tourist destination at a popular time of year. The rest of the time I have no difficulty finding motels, B&Bs, gites etc.
As for "building up" fitness during a tour, obviously if I'm spending five or six hours a day on the bike I'm likely to be fitter at the end of a tour than I was at the start, and I might avoid jumping straight into severe mountain territory right from day one. But I don't make any particular effort to increase distances or intensity through the tour - like I said, it's not a race.