michelem, you can totally do this. First off, if you can do a half, you can do a full. Physiologically speaking. It would be difficult but you could do it.

So really, you're already there and the next bit of training is fine tuning. I would second (third or fourth?) getting a coach to help you schedule your training to avoid injury. It's very easy to overtrain for something like this, especially your first one.

Have fun!!