Hello Legless,
It sounds to me as though your marathon training is catching up with you and you're getting tired. Training for a marathon is an exhausting business, and I don't think you should expect too much of yourself in other races between now and then. I don't know when Windsor is, but isn't Dublin at the end of October? If I were you I'd go out at Windsor and have a comfortable run. Don't aim for a PB - just see what comes, and look at it as a training run for the marathon.
There'll be plenty of time to set PBs for other distances when you have Dublin out of the way, particularly if that's your first marathon :-) Aim to get round without hitting the wall. If you get your sub 4 then that's brilliant, but imo it's more important in the long term to try to enjoy the experience, especially first time round. I say this having missed out on a sub 4 in Belfast, my own first marathon, by 8 minutes last May. It was a little disappointing, but I slowed down half way round when I realised that on the day I couldn't keep the pace up. I didn't hit the wall, and now I'm hoping to get that sub 4 time in Venice at the end of October :-)
Anyway, very best of luck to you, whatever you decide to do at Windsor.