There are many,many people on this site (including me) who've been diagnosed with cancer and very fact that we
are here should be quite a good indication that such a diagnosis doesn't automatically mean that you need to start thinking about booking the crematorium! (50% of people in this country get cancer). So don't start panicking too much!
I've got a different type of cancer (prostate) but I've been through chemotherapy, and pelvic radiotherapy, and I can assure you that it's probably nowhere as bad as you might think it is. (With the exception of tiredness, any side effects of chemo can usually be easily treated these days and radiotherapy is an absolute doddle).
This page, and the links from it, might help to answer many of your questions:
https://www.cancerresearchuk.org/about-cancer/bowel-cancer
Good luck!