I'm in the same situation, I've decided to do the bridge to Brisbane run in September. Its currently 12kms but there is talk of it being reduced to 10kms (hopefully it will as this is my first run).
The advise I was given was to do the c25k program which is a 9 week program to get couch potatoes off the couch to running 5kms or 30min consistantly. I'm almost finished week & 2 it has been really great. Its interval training combining walking & running. More more info the website is http://www.c25k.com
& you download the podcasts each week & listen to them as you do your training.
If you end up doing it you should join Tara & I's challenge, there is a thread in the challenges section called Cass & Taras c25k challenge. We're not far into it & are happy for anyone to join the challenge. Its great motivation & also great to have others doing the same thing you are.
Either way good luck with your fun run!!