New Runner
I also do virtually all my training on a treadmill but still love to do races. I am probably a little slower in a real race, but I have yet to really push myself.
Like all the others said, there are many, many walkers in these 5k's and not one of them cares how anyone else finishes. All ANY of them care about is finishing or raising money for their cause. There really are only a few who even care about winning! Almost everyone else is thinking about their own race.
Most 5k events really are fun and most runners are the most supportive group you'll come across. They love to see new runners go out and conquer their first. I think you'll be pleasantly surprised.
Like the others said - GO SIGN UP FOR ONE! The experience is just waiting for you!
Linn
I ran my first 5k race 2 weeks ago. My time was 39:47 and I wasn't last although that was my big fear also. Use your treadmill's incline feature to mimic hills so it's like you're running outside. I've started trying to improve my time so I run at 6 mph for 2 minutes, then walk at 4 mph for 2 minutes and do that over and over. I think it's helping too! Be strong, you can do this! Now girlie, so sign up for one!
Jules
I ran my 1st 5K in May. Signed up the day before and just did it. Don't sweat the details. You are doing something very few people (percentage wise) do. You won't be last. Just try to run your best and walk where you have to. Put your Ipod on some good running music. Relient K is really good for running. Good luck!