The other alternative is to look at a run/walk programme. You might find that helps you get around on a more evenly paced basis. If you've only done 12 miles and have 5 weeks to go, then you're going to be pushing it to get the miles in.
If you're going to do that, then start now, work out what your run / walk ratio is going to be and start training.
Don't forget to practice your nutrition/ hydration as well - check which gels etc you're using and make sure they agree with you. I'm assuming that if your longest run to date is 12 miles that you haven't yet started using gels.