I typically incorporate my fitness goals into tours, i.e. they aren't just holidays. Even then the mileage varies according to numerous other factors. In general I mostly do >100km per day, and also take note of the meters climbed. On one recent trip the day total was 100km just on, but the ascended meters was at 3500, my legs definitely told me the truth of that. So in that way, I'm more concerned with getting a decent physical workout whilst enjoying the many other aspects of a bicycle tour, but mileage is only one factor of that.
As others have noted, bike comfort is possibly the main determining factor as to whether you feel able to ride for extended periods (in conjunction with weather), so take time before the tour to make sure you have this worked out, and also don't be afraid to experiment with small adjustments as the tour continues.