It's been so long since I used the older version of Pages I can't remember how you changed fonts. Now, it's relatively easy. The ruler at the top has a font button. You tap it and choose the new font. Do it when you first open a blank document and it will be the default font for that document.
But you are right. There is no way to change the font so that new documents open with it as a default.
Then again, there's no way to change the default font in the OS X version either.
I believe this is because Apple went all in with the template concept for new documents, and templates contain all the defaults for that document. Even a new blank page is just another template.
While Tim's solution of creating documents and copying them each time is sound, you can improve on the method just a tad.
After you create the document with the fonts and/or other defaults you want, tap the wrench icon, Share and Print, then Copy to iTunes. Use the Pages format to save it. This places a copy of the document in a special area reserved for coping files to and from a computer using iTunes. It will stay there unless you intentionally delete it either on the iPad or in iTunes when the iPad is connected.
The next time you want a document with those defaults choose the Copy from iTunes option instead of Create Document. It takes the same number of steps as Create Document, so it's not a big hassle once you've set it up.
The advantage of doing it this way is that you never have to worry about accidentally editing your original, because a new copy created each time you do this, just as if you had used one of the templates.
Another similar solution is to use a cloud file service that supports WebDAV. Box.com does this, and I use it this way fairly often. The steps are exactly the same, except you use the WebDAV menu items instead of iTunes. The first time you do it you will have to set up the WebDAV service. Instructions should be on that service's site, somewhere.
The advantage of doing it this way is that you are also creating a backup copy of your templates. One that won't get deleted if something happens to your iPad. It's also a good place to save the important documents that you can't afford to lose, for the same reason. You can't rely on iCloud for this because iCloud is mostly meant to sync your documents between devices; it updates much to fast to be considered a backup. Mistake become irretrievable very quickly.
The disadvantage is that your templates are online and can not be used without an internet connection.
Personally I use all three methods in a mixed up sort of way. I keep copies of special templates in a Template folder on the iPad. If I use them frequently I copy them to iTunes (haven't done this recently), and all my important documents get saved via WebDAV to Box.com whenever I update them. I keep a template folder on Box.com as well, but that is mostly so I can share them if requested.
DropBox can also be used, but since it does not have WebDAV you have to use the DropBox app as an intermediate. Use the Open In option to save a template to DropBox. To create a new document start in DropBox instead and select the template there. You will be able to use Open In to make a copy in Pages. Other than that it has the same advantages and disadvantages as using WebDAV.
Ok, that was a much longer post than I intended, but hopefully it's given you an idea or two on how to make your iWorks workflow a bit easier.