DD, I did the GNR last year as my first 1/2M, I trained 3 to 4 times a week and did an average of 25/30 miles a week with my biggest run of 10 miles. The crowd, the atmosphere and sheer bloody mindedness carried me round and I loved every 1.53 mins of it.
Windsor will be my 2nd 1/2M and only my 3rd race, Training is going soooooooooooo badly, my body is falling apart, and I too struggle to get motivated for the required training runs, but I think that the only person you have to answer to is YOU!!
you are the one who will know after windsor if you have run a realy good race, or could have done better, so take a good look in the mirror, and decide what you want to achieve and how you are gonna do it, you still have time if you want, to reach your goal.
C U at Windsor and good luck