My buddy Dave had the same issue with his thick fantasy omnibus. He switched to supporting the weight with his palms instead of his fingers, and it made a huge difference. What kind of adjustment worked for you?
Oh man, Dave's palm trick is solid advice. For my copy of that massive Stephen King hardcover, I actually found that propping it on a firm reading pillow on my lap was the game-changer. It takes all the strain off your hands entirely and lets you just focus on turning pages. Your mileage may vary with a pillow, but it made those long reading sessions possible for me.