Don't worry - you will do FINE!
I did my first 5K LY and I was like you: I walk/run 2.4 mi every day, but I rarely ran that whole distance. As the race approached, I was getting really nervous!
The week of my Saturday 5K, I decided to "test" myself by seeing if I could run my usual route and half another one midway during that week. I did it! I just ran slow and I was amazed at how months of training paid off.
You will be, too.
Also, when you get there, you will be excited and the adrenalin w/ carry you through. And even IF you did decide to walk parts of it (I bet you don't, though), there is absolutely no shame in that - LOT'S of people do just that. I saw very pregnant ladies at my run who signed up just to walk the course; kids, older people, etc. That was the norm. Everyone is happy and having fun and excited to be there. If it's for a cause, people are happy about supporting that. No one cares who's skinny, who's walking, etc.
Like others have said, don't go busting out like a bull in a shoot; pace yourself and I promise you, you will do just fine!
Let us know how it goes. I bet you have a blast and will be so proud of yourself for doing it! I'm proud of you!!