Vanity Fair. Sometimes you will find yourself forgetting that the characters aren’t real people.
If you didn’t like the Culture books then I suggest the Algebraist, I think that is Bank’s best non-Culture book.
Another one, Dissolution by CJ Sansom set in 16th century England at the time of the reformation. It’s one part murder mystery, two parts historical drama (and if you like it there are some sequels which are also good).
Do you like Gore Vidal? I’ve only read one of this books, Julian, but I really enjoyed it (historical drama set in the late Roman period, massively scathing wit).
Otherwise I think the poisonwood bible would be a goodun, it’s meaty and should keep you going for a while.
The original Dune is long, epic, and generally not bad, IMHO.
The Hitch-Hiker’s Guide To The Galaxy: A Trilogy in Five Parts is one of my perennial favourites, as is the Red Dwarf omnibus…
Ella Leffland’s The Night, Death and the Devil is a 700 page novel based on the life of Hermann Goering. It’s very readable and true to the known facts, but allows Leffland the novelist’s ability to speculate on inner thoughts and the undocumented events that make up much of the life of even the most famous (or infamous, in this case).
Sarum by Edward Rutherford tells the history of Salisbury, England over about 1,000 pages through the eyes of a few families. I’m rereading it now and in it you catch glimpses of Britain being separated from the continent at the end of an ice age, the building of Stonehenge and why it was built, the Roman occupation of Britain and Boaddica’s revolt, and Viking raidss, and that’s just in the first half of the book. Later in the book, Salisbury Cathedral is build by a descendent of the man who designed Stonehenge. I’ve double-checked your requirements, and I think it should do.
If you liked Watership Down, you have a taste for fantasy. I’d recommend The Once and Future King by T.H. White – Superb treatment of King Arthur that manages to be faithful to Medieval conceptions and modern sensibilities. It’s the basis for both the musical Camelot and Disney’s “Sword in the Stone”, but neither manages to quite capture the spirit of the book. It’s a GREAT read, and it’s estill in print. (Actually, Disney’s movie is based on White’s The Sword in the Stone, a free-standing book that he extensively rewrote when he made it the first book of TOAFK. You’d have to look harder to find it.)
If you like it, go on to his The Book of Merlyn, which was supposed to be the last “book” of TOAFK, but was excised, and not published until after White’s death. I think you can still get this one, too.
If you like Julian, which I recommend, too, then try Creation — the nephew of Zoroaster travels to India and meets with Gautama Siddhartha’s disciples, then on to China to meet Confucius (!) More engaging and interesting than it sounds, believe it or not.
The Company series, by Kage Baker. Start with In the Garden of Iden. Stop with The Life of the World to Come and make up your own ending. Maybe read The Children of the Company sometime before or after The Graveyard Game.
Decent length reads in and of themselves, and especially awesome and long when combined.
I’d also recommend Emma Bull’s The War for the Oaks and Robin McKinley’s Sunshine. Mostly because I really enjoyed those, and we seem to have similar tastes given the above lists. They’re lighter reads, though.
Solomon Gursky Was Here - Mordecai Richler (his best novel, and a relatively long one at about 500 pages).
A Hero of Our Time - Mikhail Lermontov (shorter novel, good for one x-country flight; really rips along).
*Slayground * and Butcher’s Moon - Donald Westlake, writing as Richard Stark. The two novels go together (BM taking place several years after Slayground). A total of about 500 pages for the two. Writing as Richard Stark, however, Westlake’s prose is incredibly tight, and it won’t take long to get through these two.
If you like intelligent historical fiction you could try the both the *Lymond * and Niccolo series by Dorothy Dunnett. Well written and impecably researched they introduced me to areas of 16th and 15th century history I hadn’t even thought about.
With six and eight books in the two series and each book of a very solid length, you would have a lot of reading there. 
Herman Wouk: The **Winds of War ** and War and Remembrance. Two amazing works of historical fiction set during World War II.
I’m thoroughly enjoying Hy Brasil by Margaret Elphinstone just now. It’s set on the little visited island of Hy Brasil (well, it was known as Friesland when it was a British colony) where a travel writer of sorts goes there to write the first the first guide to the island and gradually discovers layers of secrets, some dating back centuries…
I’ve read a couple of her other books as well and enjoyed them; Voyageurs & Light. And I’ve just bought another couple of her books in case they go out of print…
Since you’re a bit of an Anglophile, I’d recommend Jonathan Strange & Mr. Norrell by Susanna Clark. It’s an alternate history England during the Napoleonic Wars where “English magic” makes a revival. Intricate but not too dense, and the plot keeps a pretty good pace.
I just read The Anubis Gates by Tim Powers and I’m running about recommending it to people. I think it would be perfect.
Actually, it’s more interesting than that review makes it sound. It reminds me of Jack Finney’s time travel stuff…hey, have you read Jack Finney?
Let me suggest Towing Jehovah by James Morrow , although it’s not 500 pages, it sounds like it’s up your alley.
I’ll also suggest Gun, with Occasional Music by Jonathan Lethem. Also perhaps not quite as long as you want, but it’s really unique.
Finally, in the “weird things happening to ordinary people” category, I’ll nominate House of Leaves by Mark Z. Danielewski . This book is a complete mindfuck. I’ve never read anything quite like it.