My guess is you'll still be tweaking it as you go along.

There are some larger factors still missing such as the actual weather and logistics/supplies as you go along.

So I view my initial planning/reading as creating a menu for later rather than getting too caught up in doing a lot of tweaking in advance. For example, a choice of whether to bypass or go through a town I might make based on whether it is raining or whether I camped somewhere and need groceries or to stop past a laundromat or whether I accidentally left my flashlight behind, or whether there is suddenly road construction changing dynamics, etc. All situations I might not know until the actual trip.

So what I do in advance is more like anticipate:
- are there areas with larger gaps where I might need to plan my lodging
- what areas seem to have services
- what are some features of "must see" places on my candidate list to visit or people I visit
- how will I replan if needed e.g. on phone, library PC, etc
- do I have major options if needed, e.g. train

During the trip:
- I talk to people along the way and other cyclists, sometimes adjusting again
- I get info about real time conditions and new things (construction, hazards, weather)
- I still keep my route somewhat dynamic
- I have fun
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