Try to get something a bit longer in, even if it entails a little walking, just to remind you how it feels to be on your feet for 2 hours. Try 30 mins run/5 mins walk/30 mins run this weekend. After that, continue with your 7ks, maybe doing one or two at a faster pace, so that your current speed feels even easier.
On the day, concentrate VERY hard at the start, particularly if it is quite a small field, on running at your own pace. My second ever half, which was a small local one, contained a lot of pretty good club runners. My usual 10min/mile max pace was replaced by a first mile in under 8 minutes because I was trying to hang on to the back of the pack. After 3 miles I was suffering and by 10 I was walking.
Good luck, and let us know how it goes.
