Wow. I am less than a week out for surgery to repair a running injury, and I have been absolutely freaking out about the distant-but-theoretical prospect of never being able to run again. Thanks for helping me reframe my priorities.
My father lost both of his parents by the time he was five. He treasures pictures of them, books that belonged to them as children, contact with other relatives who remember them and can tell stories about them. Whenever he's learned about something they were interested in, he's pursued it; he took up model railroading after learning that his father loved trains. And he has a very few pictures of them with him that he probably values more than anything else. When you are doing both the big things and the small things with your wife and kids, make sure there's someone with a camera around. Those pictures will mean everything to your kids, whether you beat this thing or not.
One summer, while visiting a relative, we found a stash of love letters my father's parents had written to each other before their marriage. The whole family devoured them, reading them aloud. If there are things that reveal what kind of person you are--old letters, running journals, even threads on letsrun.com--make sure your kids know where to find that stuff. They'll want to know everything they can about what you were like, not just as a dad, but as a person.
I wish you well.