Not sure if you can say the same for Walt. Swiss Family Treehouse opened on November 18th, 1962. Disney released their film "Swiss Family Robinson" on December 21st, 1960. The attraction was put up in less than 2 years after the film was released.
I think we just have to remember that Walt's real vision was not for a whole slew of theme parks in Orlando. He already did the theme park bit in Anaheim. He wanted what we know as Magic Kingdom to be a money source for his vision of EPCOT. He was pretty much bored with the theme park idea, and just wanted Magic Kingdom to draw tourists/citizens to what his EPCOT was going to be.
Also,
Disneyland used to be closed on Monday (and maybe Tuesdays...can't remember). So that allows 48 solid hours of construction, without any guest interference. It was probably a lot easier to put up attractions back then.
If anything, imagining someone like Walt was in charge with today's scenario (multiple parks), I would argue that he would be pushing as much IP as possible. I don't find it hard to believe that imagineering had two or three ideas for every single movie released by Disney at the time (prior to the film even being released). If the movie was a success, I'm sure giving the greenlight to one of those ideas was pretty quick. Get the people in the parks and let them visit the worlds that they loved from their TV and movies. Once you have the next box office hit, replace/add/overlay/etc.
Money baby.