Have only skim read the link article and a bit of the one referred to in that from ComicCon.
Thing is there is no 'book' to model the prequel/concurrent storyline to this which isn't just based on Appendices to Return of the King but also to other notes which are cobbled together in one of the later books in a series of 12 that make up the marathon called The History of Middle Earth
which is is actually Christopher Tolkien's archive material of his father's notes going back to before his parents were married. So it's certainly more than 125 pages of manuscript notes which are exactly that and not even as polished as some of the sketchier parts of the Silmarillion.
The 'extra' film is partly from the LoTR appendices and the rest from the part of HoME, Vol 12 Peoples of Middle Earth
and is a typical Tolkien 'arse about face' piece of backtracking
as the LoTR piece was done first and fills in the bits of The Hobbit
where Gandalf swans off leaving the dwarves up to their ears in Woodelves and Spiders and the latter was done afterwards, and before he started in on The Silmarillion
in earnest that deals with what happens before The Hobbit
when Gandalf discovers Thorin Oakenshield's (Richard Armitage) delirious and dying father in the Necromancer's dungeons and manages to liberate the map and secret key that sparks the whole adventure off.
So I imagine what's happened is that, because there's no book but plenty of notes and outlines knocking around, Mr & Mrs. Jackson have been embroidering long and hard on the 'not properly told' stories before the momentous party and then the parts that weren't dwelt upon at all in the several months in between Gandalf leaving the Dwarves and Bilbo on the western side of Mirkwood and then popping up again in time for the Battle of the Five Armies at the end of the Hobbit
. Both add-ons are dark and ominous in that they deal with an 'about to go into his flaming eye phase' Sauron terrorising the southern parts of Mirkwood and generally making life hard for Galadriel and Celeborn over the river in Lothlorien and for Legolas' daddy in the north of Mirkwood. The Elven backstory's quite a potboiler as it brings up a load of bad blood involving Celeborn's and Thranduil's (Legolas dad) family and may throw some light on just why Legolas and Gimli hate each other's guts to start with...
In a way it's the story of power of the Elven Rings as well, as the battle that Gandalf (and his cousin Radagast) has with the Necromancer is largely one of power wielding by Galadriel, Elrond and the Wiz (as the 3 ring holders) and also the beginning of Saruman's treachery, so a lot potentially happens. It's certainly a much more adult-oriented tale with the earlier part with Thorin's father happening before Bilbo was only a very small and infantile hobbit
So maybe not quite Twilight or Deathly Hallows carve-up territory, but there's likely been a lot of extra and speculative footage kicking in so there should certainly be enough for 2 very long films - I guess we'll have to see how far they get in this first film come Xmas to gauge how much more they've got to mil... get through...