Hi Donna,
Your post and some of SlogOn's funny stories about approaching one's first 5K, in some other threads in the Running forum, have me really happy to have had the experience of the little fun runs I've participated in. Not that they are super-orchestrated, but having now done a couple I feel totally comfortable at least about a few things:
race bib - pinning it on with safety pins works fine on most singlets (sleeveless running shirts made of performance fabrics), probably also most other fabrics. I wore a cotton and spandex blend yoga tank for my first run and the pins worked fine; for my second I had a Champion singlet on and the number still worked just fine. My club supplies the bib WITH safety pins on it. Oh -- the first night, I was walking around cooling down and eating the ice pops they hand out at the finish, and nearly forgot I had the big still on. My club collects the bibs and reuses them for this series of races -- I don't remember whether they do this on most 5Ks or not, but if so, remember at the end to turn it back in with the safety pins!
heat - you already know to hydrate well, starting now and through the event and after. I took a plain old bandana with me the other night. I rolled it into a tube and tucked one end in the waistband of my shorts. The humidity turned out not to be as awful as my first outdoor run; still, near the end I pulled the bandana out and it felt good to mop my forehead and neck with it! I fiddled around with tying it like a headband/sweatband, later on, but I think the first option works fine for my runs. I also used a tip that I found in some article online - put my cell phone inside a snack-size baggie. I still haven't found a belt pack or shorts with pockets or such, but I like having the phone along and this way, I don't worry about sweating on it!
nerves -- yeah, in my experience, unless you are SUPER careful or use tech gear proficiently, you're likely to get caught up in the group's pacing. I know I do. Not a fatal flaw, I think we will all probably get used to it with more races run. One article mentioned that nerves actually are helpful in that they do help us push enough to do our best, and being on edge can be a slight bit of an advantage -- it's GOOD to be nervous. Eat lightly, don't eat any new foods. And, DO try and run it for the fun of it!! That's probably why you started this stuff, in the first place?
other runners -- it really is true, other runners are extremely kind and supportive. I told one woman it was the first time out for me and she confessed she likes to run short races while her husband does the marathons and all that. Greet people you pass kindly; they may just pass you further along the race! I ran passing and being passed by a couple of runners this last race and we were sort of joking with each other about it.
You will do great! Center yourself before the start, don't forget to cool down and to stretch afterward. Run well!
Linda