Okay, I just saw your preferences... I guess it's hard to write more than 500 pages about someone having a downward spiral with drugs or an eating disorder. There ought to be some long books about dystopia, but I can't think of any. Here's my best try:
The Stand by Stephen King. Long but easy to read, very absorbing. It's about the few people left after a flu kills most of the world. This is the unabridged version, but it was first published with a lot cut out. That version is still over 500 pages. http://search.barnesandnoble.com/The-Stand/Stephen-King/e/9780385199575/?itm=2
City of Golden Shadow is the first book in the Otherland series by Tad Williams. http://search.barnesandnoble.com/City-of-Golden-Shadow/Tad-Williams/e/9780886777630/?itm=1
Dune by Frank Herbert It may not sound like you'd like it, but check it out. It's awesome. http://search.barnesandnoble.com/Dune/Frank-Herbert/e/9780441172719/?itm=1
These were what I listed before I saw your preferences.
Herman Wouk is an author whose works are sometimes read in literature classes. A few of them are over 500 pages, and they're very absorbing to read:
The Winds of War http://search.barnesandnoble.com/Winds-of-War/Herman-Wouk/e/9780316952668/?itm=1
Marjorie Morningstar http://search.barnesandnoble.com/Marjorie-Morningstar/Herman-Wouk/e/9780316955133/?itm=1
Colleen McCullough's The Thorn Birds is well respected. If you like great love stories, this might be the one for you. http://search.barnesandnoble.com/Thorn-Birds/Colleen-McCullough/e/9780380018178/?itm=1
Most of Leon Uris's books are over 500 pages. Here are two:
Exodus http://search.barnesandnoble.com/Exodus/Leon-Uris/e/9780553258479/?itm=1
Trinity http://search.barnesandnoble.com/Trinity/Leon-Uris/e/9780060827885/?itm=4
Scroll down each link page for the book synopsis. The page also shows the page count for the book.