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!!
Living life like there's no tomorrow.
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