Carl,
Once you have one file adjusted to the Landscape layout that you want, you can use it as your template to adjust all tthe files in your book to match.
A quick & dirty way to make a re-usable template for the orientation is to save the file that you want to use as the template under a new file file name, e.g. Save As > myLandscape_tmplt.fm.
Then with the template file open, you can blow away the contents so that it isn't a distraction: simply press ctrl+A to select all in the main flow and press Delete. Now you can resave this and you have a simple landscape template. Keep this template file open.
Open the book file that contains the files you want to modify (tip: if it's just a collection of files that aren't specific to a particular book, create a temp book to hold all of these files). Now open all of the files in the book by holding down the Shift key and then click on the File menu and select "Open All Files in Book". This will open all files in the book in one go.
Now using the book file, select all of the files that you want to change to Landscape.
Then using File > Import > Formats... select the Document From: to be the landscape template file.
Now you have to uncheck all of the options, except for "Page Layouts". You may (or may not) want to also enable the Remove format/layout overrides (depending upon what you've done in the documents),
Now click the Import button to apply the landscape pages to all of the documents.
You can now examine the individual documents, since they are open, to see if the layout is to your liking. Note: if you don't like what you see, simply close the document(s) and you'll be back where you started from (you can do the shift + File > Close All Files... trick to close all files in the book). Had you done this Import step directly into the book selections without opening the files first, you wouldn't be able to back out in case something goes wrong, as FM opens, imports and then saves and closes the files when done from the book file alone.
Hopefully, this will make things easier and quicker.