Actually there are a lot of people who run/walk marathons. Jeff Galloway has long been a proponent of this approach. From your post I'm assuming that your long run of 14 miles has been all running with no walking? Is that the case?
What you need to do is mix walking and running right from the very beginning. You might have to experiment with this as much as you can in the time left before the race to see what works. In your case you might go three minutes running and two minutes walking. Or perhaps a 50/50 split. But do this right from mile one. If you wait until you are tired from running to start walking you won't be able to maintain that 15.5 pace you think you can. By doing the walking breaks from the beginning you might stand a chance of being able to run across the finish line in reasonable form. That looks a lot better for the race finish picture!
Here is the url for Jeff Galloway's website where you can find tons of information on the run/walk approach.