I have jumped into a 5k unprepared before and the race atmosphere always carries me further than I thought I could go!
My advice is to take it easy for the first mile - jog - and then reassess how you feel. Maybe you feel great and can pick it up a little. Maybe you feel lousy and need to walk for a minute. Don't push to hard on the front...you can always push harder at the end. Try to finish strong...that always makes me feel better even when I have a lousy race!
You can do it! You have a fitness base and your body will rmemeber how to run!