I am really sorry to hear this too - I had to quit my first marathon try because of an injury, though I wasn't quite as far along as you.

First, a word of encouragement - even though I was very disappointed to have not made it the first time I tried to train, I did come back to complete 2 of them within a few years time. So if you do need to stop this time, your motivation will be even stronger the next time around, and you'll know much more about the best way for you to prepare. (I was the one who previously recommended pool running as an option.)

Secondly, just a slight caution about continuing with an injury - even though your doc has advised that you won't make this problem significantly worse by running on it, the pain means that you probably can't avoid changing your mechanics to make the pain as bearable as possible. Altered running mechanics can significantly increase your risk of injuring something else.

No matter what happens, be proud that you've made it this far already!