I agree with K-Lo. I'd bet good money that John Edwards is staying in the race in part because Elizabeth wants him to. Of the two, she's undoubtedly more politically active, committed, and thoughtful. And whether these are her last years or not, I don't think it's a particularly strange desire for her to want to spend them making a mark, listening to others, trying to make manifest her and her husband's vision of what America can and should be.
There's a sort of subtle insinuation that sick people should crawl back into their caves and stay there till they either die or get better. But when you hear the Edwards's discuss the idea that her cancer is now incurable, that it's not something she will get better from and so not something where they can hit pause, wait for it to pass, and then resume their lives, you have to think that the question they're asking themselves is not how can Elizabeth best get well, but how would they like to spend the rest of their years. And knowing her even casually, I'm not surprised to learn the answer is "fighting."