Stick with it DS, there's plenty of time to build up to the GNR. I've had a very patchy few months but have got back into the routine again now, and while I'm still struggling a bit, I'm confident that I'll build up some endurance pretty quickly now, and lose a few pounds too, which will help a lot.
RD is right. If you've run before, you'll get back to where you were more quickly than if you were starting from scratch.
When you sail past me in the last mile of the GNR, you'll remember these words!
Good luck all.
Andy