There is a science fiction novel I read in the 1980's, and I think it was a new title in that decade, although I am not certain about that. It was the story of a fat guy who had saved for a luxury space cruise vacation and had rented a sexy, fit body to occupy for his trip. After boarding the cruise ship, he discovered the body he was given by accident was a special combat model, and people thought he was someone else. They were chasing him to interrogate and/or kill him, and he had to rely upon the combat reflexes of the rented body to keep him alive, while trying to figure out what was going on.