Congratulations on getting in to do the GNR. I did it last year and it is an awesome experience, one you will remember for a long time. If I can give you some helpful advice on the run -
Don’t expect a quick time! There are way too many runners, just enjoy the day, enjoy the run and most of all enjoy the support you get from the crowds - they are amazing. I did GNR in 2:37 and then did Leicester Half 4 weeks later in 2:17.
Take something warm to where at the start – something old to discard before you get over the start line. You can be waiting at the start for 30 -40 minutes.
Save some energy and enjoy the last mile. You will have never seen crowds so willing to shout your name and cheer you on, a fantastic experience.
I would agree with Nelly P on your long runs – if you can get up to 17K you will be fine. Take it easy at the start and try to get into a comfortable rhythm. All in all just enjoy.