It takes a lot of time. I don't just make this stuff up. Everything is thoroughly tested, in some cases more times than any of us would like to recount. We've tried a whole bunch of pretty bizarre ride combinations too, "just in case." Then the actual writing of the plans takes a lot of time, especially with the full map in Adobe Illustrator, menu choices in Adobe Acrobat, full writeup in Microsoft Word, and then the conversion of all three into the final PDF file.
What would be really cool one day is if they could be personalized to take information like the day visiting (or crowd calendar), then use the estimated wait times/ride times/walk times to give an estimate of what time you would be done with each ride or at certain points in the plan. Not sure that can be done without a spreadsheet or database included in the process, but it would be cool if it could be done.