I'm so glad that you've decided to keep her appointment to be spayed. It really is better for her, believe me!
My old girl was probably about 10 years old when I found her, and unspayed. I had her spayed of course, (they say the only reason she wasn't pregnant is because she was too thin to carry a litter!
) But since her previous owners didn't spay her, and she was older when I found her and did it, she's now having a mammary lump removed and it's suspected she might have mammary cancer.
Better to get it done now than to regret it later!