There's a story I read awhile back (here?) where in the beginning the government(?) is testing a cure for some disease, and only a man and woman survive the testing. Turns out it also turned them immortal.
Soon after they survive the fall of civilization and by the end of the story mankind is just heading into the stars.
There was a far future postlude (thousands of years after the last chapter) where it is strongly implied that the two people still meet every two or three hundred years.
Anybody remember it? For some reason I want to say the title is something like "Forever and Always", but I haven't found it.
