I would say YES...you can do it!!
Loads on hear no more than me regarding running/training etc bu toff the top of my head i would say to you..
(BASED ON RUNNING 3 TIMES PER WEEK)
If 5 mile does you in now, do 2 x 3-4 mile runs in the week and a 6 mile run/walk a bit run on the weekend .... Build it up 1 mile per week so in about a month you will be on a weekend 8mile run and 2 x 4-5 miles in the week.
If you can do this after a month.....either stay as you are miles per week wise if your struggling or increase your week end run by 1 mile per week.
By the time GNR comes round your be 'OK' and will enjoy it because the 'EVENT' its selve will give you a extra push on the day.
All my opinion wich means bugger all!!!!!!!!