I probably read this in the 1970s, and I may possibly be conflating several books.
I think the story begins with the hero (who may not know who he is) either killing someone or witnessing someone being killed with some horrid poison which turns blood into jelly in a few seconds.
Continental drift may have led to a single supercontinent (but I am pretty sure this is not the Lin Carter books).
There is then some complication where he visits a city (or city-state?) and is sent on a mission of some kind perhaps to recover or discover something, probably with others.
And the bit I remember most clearly: they arrive at the ruins of some fantastically advanced city which has been destroyed, eons ago, in a conflict with another city or state: there are buildings with holes punched in them by who-knows-what weapon, and there may have been flying buildings which have crashed as the inevitable antigravity devices failed long ago. Some may still be flying. Exploring this place they come on a person or people who was in charge of the city at the time of the conflict (so is vastly old) and does not understand that the conflict ended thousands of years ago.
I don't remember what happens after that.
I have looked through a bunch of 'dying Earth' lists but can't find it. It may be my memory is just entirely faulty.