That really is a shame!!! You don't necessarily need to pull out though - although I can it depends on your feelings of how a race should be treated.
If you're recovering/recovered, then although you've not ran the distance before, you could still use it as a training run, and an experience to get you used to the whole 'race atmosphere' even if you say, ran 6miles and pulled out there, or ran 6, walked 1, ran 6... ran 10 and pulled out (or walked to the finish). There's a whole host of ways you could still stand proud on the start line, but just adjust/forget any goals you'd originally set yourself.
I understand for your first event though you may want it to be a memorable and happy experience where you run start to finish untroubled and within your goal time - so I respect your decision if you give it a miss
