I heard once, from a fairly reliable source, that cancer grows quicker in young people. Cancer is much more common in old people, though. There it grows more slowly, and presumably also spreads more slowly.
So take those "5 year survival" stats with a grain of salt. They're based on averages, including the average at which people get that particular kind of cancer. If someone gets it much earlier, 15-25 years earlier, being alive 5 years later is much less likely (unless quicker growth leads to earlier diagnosis of course, if earlier diagnosis means treatment).
The spreading, called "metastasizing" or something like that, is often what causes the real problem; the first tumour, the original one, might quite likely be in a fairly harmless place, just an annoying lump, maybe eventually pressing a bit on some nerves or something. But if single cancer cells, or small bunches of cancer cells, get torn lose (which happens frequently with some forms of cancer, not so often with others) and spreads to other parts of the body, and manages to grow there (i.e. gett enough nutrient-rich blood), then chances are one of those dozens of colonies will sit somewhere important. Squeeze on something that dislikes being squeezed, or maybe close off blood supply, or render a nerve nonfuctional.
Also think about what kinds of pain killers is the character on, and what side effects they tend to have. If your setting is technologically primitive, maybe they only have primitive stuff like morphine (which, by the way, can easily be used as assisted suicide, if the patient is given extra-high doses).