Only joking Debs!
As you already have a few races of 10K under your belt, a "Half" like Cardiff should be a good introduction to the longer distance.
If you can try and get a long run in once a week,10 miles if possible,and 2 or 3 runs of up to 5 or 6 miles you should be fairly comfortable for the big day.
The main thing is to train sensibly,set yourself a realistic time,go out,do it and,more importantly,
enjoy it!
Enjoy life! Enjoy running!