Most people doing the GNR are in the same position and it's just a question of adjusting your training plan. What you should not do is come back from holiday and do a 13 miler two weeks before, as a newby that is not enough recovery time.
Also consider if you do not get a place in the ballot try the local charities - St Oswalds, Percy Hedley, grace House. All you need to do is raise a certain amount in sponsorship, Percy Hedley was £250 last yea, the bonus is they all provide free refreshments in the charity village afterwards and great support on route.