There's Stephen King's
The Stand. The revised edition clocks in at 1,300 pages. The beginning of the book, the (apropos of the time) outbreak of a killer flu that wipes out most of humanity is pretty horrific. But King's goal was to write a modern day Lord of the Rings-style fantasy in contemporary USA and I think he did a good job of it.
A stand-alone fantasy I read decades ago is
Villains By Necessity by Eve Forward. It's set after the end of a typical fantasy story where the heroes have won. An assassin, a thief, a sorceress, a dark knight and a druid set out to bring evil back into the world. It's tongue in cheek and not a masterpiece, but I enjoyed it.
For sci-fi, I liked
Stephen Baxter and Arthur C. Clarke's The Light of Other Days. A novel where they introduce a new invention (a wormhole camera that lets you see anywhere on Earth and eventually into the past) and explores all the rippling changes that has on society.
I also enjoyed
Poul Anderson's Brain Wave. Earth passes out of a magnetic field that suppressed intelligence for eons. It's short, but there is a lot to think about as you read.