I agree with what the others have advised. Your daughter may be hearing the whispers in the family anyhow, and wondering what's going on. It may be worrying her.
So let her know you're going into the hospital for surgery. Emphasize that you're not ill and your life is not in danger. Let her know how long you'll be in the hospital and how long it will take for you to recover when you get home.
Encourage her to ask you *any* questions she wants, even if they might be "dumb" - you know, sometimes the "dumb" questions are the best ones. She may not ask anything right away; however, if you leave the questioning door open, you may get them over a period of time. That may be one reason to talk to her about this now. You know your girl well enough to know, probably, whether she'll ask out loud what she really wants to know. If she is a good asker, you can wait to go into more detail until she wants the detail. If she keeps her questions to herself (I was like that as a kid), you may have to do some of the explaining anyhow to ease what you know is in her mind.
I don't know your motives for the procedure, so I'll make some up: "You know I've been getting healthier, and I've lost a lot of that fat, which has been good, and I'm very thankful. But when you lose fat, you don't always lose the skin you need to lose - like THIS skin. Sometimes that has to be done by a doctor. Dr. Smith is going to..." and you explain briefly what will happen to you... "I'm going to look very weird at first and be very sore, and it's going to take me several weeks to get well, but then I'll be able to hike more and ride a bicycle and..." and you list the real benefits of the procedure.
Believe me, when she sees you right after surgery, "glamour" is going to be the word furthest from her mind.