I think it really depends on how far outside your comfort zone you are
Yes, I think that really is the deciding factor. I'd quite happily take off on a tour of France tomorrow, without any form of research/prep. Just what we'd call in NZ a 'ticki tour. The same would be true for Western Europe, I'm comfortable enough with the environment to go pot luck. But for Eastern Europe, there's a few things I might read up on. More so for South America etc etc etc.
Danny, I'm fortunate enough to not need to set targets. I could go on an open ended basis. I'd just ride as I felt, no daily mileage target.