Do it! Exercise has been shown to benefit cancer treatment in many ways. You'll feel better for it.
The 'I'm going to die syndrome' is a toughy. We all are obviously, it's just a matter of timing! A cancer diagnosis brings it all a lot closer, but statistically it's unlikely this will kill you. Try to breathe through it when it threatens to overwhelm you, and to put it out of your head as best as you can. This feeling will get better as well, but it takes a while.
A lot of people told me to be positive, to stay positive, that it was key. People who barely know me (like a check out chick at Coles) and friends alike, and it drove me nuts. I'm happy to say not one medical person did. If you can be positive, it's wonderful and can smooth your path through treatment. However don't force it or beat yourself up if you can't. Psychologically it's healthier to go through your pain than avoid it. So cry, rage and howl at the moon if you need to. K xox