Yes, you can. You can definitely finish.
My advice: Start right away to build a good base of strength training, XT include stretching and Yoga, and maintain your half marathon running status. I am a John Bingham Marathoning for Mortals participant. As the official training season begins later this summer, you should have a good beginning point with conditioned, not over trained, muscles. During the mileage build-up toward the full you can do back to back weekends. I have done every other weekend from miles 12 up to 20 (5 back to back) and I have done purely marathon training. It's all dependant on your body's ability at the time. Listen to your body and do what you can. Get out there and put in the time. Have fun. Check back, there will be others enjoying the journey with you and we'll all love company sharing it.