My experience is a bit different than most of the other comments.

First off, I don't camp or carry camping gear. This means I have to find places to stay every night, either hosted or paid. I spend a lot of planning time finding places along my route (something having to alter my route) where I can find a room for the night. So far, I haven't reserved (paid for) a room I didn't use, though many places will allow someone to cancel a room reservation the day of or the day before scheduled arrival.

While I don't always plan every day of my tours, I often plan the first week just so I know how the tour will start. Many multi-week tours are planned down to the day, which helps me as I like having a goal for the day and knowing how far/hard I have to ride.

I have found that my tours are more enjoyable when I plan a non-riding/rest day for every 4 riding days. This isn't a rigid rule, but one I try to follow. I also try to plan routes where every fifth day I am at a place where spending a day off the bike will provide something interesting to do. That said, I can usually find something to do most anywhere for a day.

As Pete does, I also occasionally plan a short day instead of a rest day when the distance between 2 towns is too long/hard for one day and there is a place between them to stop for the night.

Another way to look at rest days is as a "bank" that several others above have noted. I usually add an extra unplanned day or 2 at the end of the trip to allow me to change my schedule if something comes unexpected comes up. Generally, the places to end tours are big cities with airports, so finding something to do in, say, Vienna, Jasper, Avignon, or Bern to fill a day or two isn't hard.

Note that I have been retired since I started touring, so my schedule is somewhat flexible as to length of time to tour. Though, my endpoints usually coincide with family commitments.

As for training at home, when I am preparing for a trip, I try to ride 2 days out of three. When I am not training, I still try to ride 2 days out of three, but the distance is usually shorter.