As PP said, at this point spaying wont lessen any of her chances for cancer, so that means you have to take it upon yourself to be extra vigilant. Giving her checks at home, while you can't diagnose anything you can find a lump before it becomes worse. I would speak to your veterinarian about pyometria, and what to look for. She is prone to pyometria simply because she has a uterus, this happens when bacteria etc. makes it's way into the uterus and starts an infection. An overwhelming majority of females left unspayed develop pyometria at some point in their life, and there's nothing the owner can do to prevent it other than spay them. I don't mean to sound as if I'm putting any blame on you, I'm not, you didn't abandon her or leave her in-tact. I just encourage you to bring these things up with your veterinarian, and make yourself aware of what to look for.
Thank you for adopting this old girl and giving her a warm and loving home. Good luck to y'all!
Edit: This may sound gross, but I would become familiar with her heat cycles. When I worked at a vet clinic we saw an older dog come in with the complaint of having an extra heavy cycle, she ended up having pyometria. (The owners didn't seem to notice that she'd also dropped 20 pounds, and was so dehydrated her eyes were sunken into her head, but that's another story)