15 km = 9.5 miles and 11.2 km/hour = 7mph(divide by 8, multiply by 5).
This means that you are doing roughly 8.5 minute miles, which is a lot faster than me!
If you can comfortably run 9.5 miles, the other 3.6 will be no problem, as the excitement of the event will be enough to carry you through. However, as you already realise, there is a difference between treadmill and road running. On the road, you have to propel yourself forward rather than just up and down, and the surface is much harder on the legs.
You have plenty of time to make the switch - try to do 2 runs a week on the road, to get used to the bashing your legs take, and if you don't already, set the incline on the treadmill to 1.5 to better simulate the extra effort needed. Don't do your longest run on the road - you could get injured, which you don't want at this stage.
I'm sure you'll manage the race just fine.