You won't be the slowest, there will be at least one tail runner behind you. These are good runners who volunteer to bring up the rear and check no one is left behind.
You will have a great time. I'm normally about 180 or 190 out of about 200. its fine, it really is. You will be running alongside people of your own approximate speed, and it is all about setting your own time and beating it.
our park run has walkers, pram pushers, dogs,children, and all sorts involved. I am regularly beaten by a man who walks the whole course at exactly 5 miles and hour, while I run! I'm also beaten every week by a six year old called Melanie, who always looks like I could over tke her, but I never do.
Honestly, it isn't about who is last, and it isn't much about who is first either, although it matters quite a lot to the first 10-20 who get very competitive. For the rest of us it is just about getting round, and feeling as if you have achieved something. It doesn't matter if you stop and rest, or walk or anything. The only thing that matters is can you shave a few seconds of your personal best next time!
Enjoy.