The River of Dancing Gods by Jack L. Chalker

Beyond the Sea of Dreams... Life had not been kind to Joe and Marge. Now, according to the stranger who met them on a road that wasn't there, they were due to die in nineteen minutes, eighteen seconds. But the ferryboat that waited to take them across the Sea of Dreams could bring them to a new and perhaps better life. There lay a world where fairies still danced by moonlight and sorcery became real. Joe could become a might-thewed barbarian warrior. Marge could be beautiful and find her magical self. And it was even as Throckmorton P. Ruddygore, the strange wizard, had promised. But there was a great deal more, as they soon learned. This was a world where Hell still strove to win its ancient war and demon princes sent men into battles of dark magic. It was a world where Joe and Marge must somehow help prevent the coming of Armageddon.