ime all races are friendly and have all sorts of shapes and sizes of people and standards. (but there are some - hilly and hard ones generally - that attract more hard core runners - so it is worth finding a review). it's very exciting doing your first one - you'll never recapture the same thrill again! my first race was 8 miles and I loved it.
so no, you won't look out of place - or unfit. I think I can (now, finally) count myself as a keen and consistent runner, and as fit - but I don't remotely have an "athletic" build. I'm short and not at all skinny or sinewy.
what distances do you do at the moment? they say the basic rule is that you can increase your weekly total safely by 10 per cent - however I've read (and agree) that at the lower end of the spectrum you can increase it by a bit more than that (but possibly not if you're overweight, over 40 and new to running and various other provisos). I've upped my mileage by more than that in the past but I've only done what I felt comfy with. this article's quite good.
check out runner's world too - lots of interesting articles and also training schedules there.
definitely do yourself a schedule - if you're anything like me you'll stick to it out of fear. a race is definitely a real motivator.