Such a bad idea.
I speak from experience; I once ran a marathon on ZERO training... I had signed up for the race well in advance but was plagued by health/life problems in the months leading up to it. Needless to say, I shouldn't have run, but I did anyway... I ran 4:38:11 (deathly slow for me, I have a 36:00 10k pr). The race hurt like hell and I had to walk several times. In the weeks following the marathon, I experienced pain in my leg and was eventually diagnosed with a stress fracture which took ages to heal.
In conclusion, don't do it! Even if you end up losing money because you've already signed up, you need to think about your health. Seriously. Start training and run one next year.