Forget your previous 5k time, that was when you were running more regularly. For this particular event, consider it an opportunity to set a time now, to improve upon in the near future for the next event after that. A baseline, if you will.
Don't kill yourself. If you need to walk some, do.
While Heather's right that if you're not consistently doing that distance now you won't magically do it in 2 weeks, you do find that "race day enthusiasm and atmosphere" pulls you through *some*. If you're able to run for 25 minutes now, you can probably complete the race fully at a run. If not, you'll probably walk some. That's okay - you're only setting a baseline.
Deb, in New Zealand