Recent hard SF? Got me there...

I just read a ten-year-old Greg Benford novel (Great Sky River) that I picked up on a whim -- got a third of the way in before realizing I'd read it once before. Finished it anyway, but understood why I didn't remember it.

But try the "Baroque Cycle" books by Neal Stephenson. Since you've re-read LotR, your eyes are obviously up to the challenge -- and by the time you finish the second one, the third one should be published and available. Cryptonomicon and Snow Crash also by Stephenson, are also excellent.

Since you like Niven... have you read Destiny's Road?