Many times it's not the cancer that makes people look and feel sick but the treatments! I"m a breast cancer survivor, and remember saying to the doctors a few months post-op and during treatments (radiation and hormonal chemos), "I felt fine until I came here and got treatment!" and the doctor replied, "That's what they all say." Not exactly funny... but sort of true.
You should be extra thankful your mother is in good shape because that makes treatments easier, and she'll tend to stay healthier...there's something called a Karnofsky score and it's a scale of 1-10 that defines peoples' performance score, how active they are. The higher the score, the better survival people get. Your mother sounds as if she'd score 100%, and that's a very good thing.
A series of 9 radiation treatments won't give any bad side effects, but should definitely shrink the cancer. Do you know what kind of chemo drugs she's going to recieve? Some are easier to handle than others, but your mother sounds like one of those people who might just get through without difficulties. There are always ways to combat side effects before they get too bad, as well.
That's great you're planning to take over some of her holiday preparations..I wish the second generation in our family would start, as me and my older sisters are getting too nutty about it all! We're between 50 and 60, I find the older we get, the more stressful organizing get-togethers becomes. I'm sure she will be thankful to see a tradition being carried out by you this year.
Please stay positive! This isn't the time to be morbid, but to be supportive and yes, hopeful...and keep the faith too. I know how hard it can be when you're worried about someone you love, there's nothing worse. But that won't help her OR you.
There are people out there who really do beat the odds and get many years of quality time after a dx of metastatic cancer, I've met them. She might fall into that category, I hope so for you.
All the best,
Wendy