I ahevn't had any surgeries that long. But I have had several of them that were 4 hours or more, and I want to reiterate what others have said: you won't know how long the surgery is when you're under and you won't be allowed to feel much pain after. I think every surgery I've had, the first thing they ask when I have come out from under is whether I'm in any pain. And if you answer yes, they do something about it immediately!
Every time I've had surgery, I've been scared beforehand, until they put me out, but I always get this rush of relief afterwards--it is almost like some kind of high (probably aided by the pain meds) that I've gotten through the surgery and it's gone well. I bet you'll get that too.
The last surgery I had (a couple of weeks ago), I would just tell myself when I was scared that pretty soon it would be over and I'd be feeling the rush I got from the relief that I got through it and it went well!
You can get through this. I know it sounds terrifying but it's not nearly as bad as it sounds.
Nelie