Jen21,
I know it sounds like preaching to the choir but, by now, you have realised that the marathon distance ain't easy. What you have behind you is the hardest bit of the whole process---the training. All those miles on your own, through the winter months will pay off. The wet, cold, windy, dark nights----my advice would be to never forget them . If you can get through that you can get through anything.
The beauty of the London Marathon is that it's run in mid spring. The weather will be a lot warmer than a Scottish West Coast winter---and no wind!!!! maybe just a wee bit of light drizzle though, to keep the runners cool--ideal!. There will be thousands of running partners and you will be supported from the start to the finish by the crowd. Just watch your pace. Be careful not to get drafted along too fast at the start. If the unlikely happens and, at the latter stages, you find yourself struggling mentally (it won't be physically, cos you've done the prep) just hark back to your hardest training run and remember how you got through that. Don't be scared about it===enjoy it.
I remember the Glasgow Marathon in 1984, I had done 3hrs 2 mins the previous year and, with less training, thought I could have a go at the 3 hour mark. I saw this group of runners go by me, yes my pace was all wrong, this lot were behind me up to 2 miles!!!!
Anyway in the middle of the group was Leslie Watson (Google her, she was Scotlands best female Marathon runner in the early 80s). She could run somewhere in the 2 hrs 40 to 2hrs 45 mark, so I tagged along with them--up until the 10 mile mark. We went through 10 miles in 62 minutes. After 13 miles I cramped up in both legs. I eventually finished in 3 hrs 25 mins having had to stop every couple of miles to let the St John's Ambulance boys do a quick massage and send me on my not so merry way. That was the second hardest thing that I have done. I really had to dig deep into the mental reserves cos the physical ones had packed in.
Jen, that won't happen to you. You have prepared.
The training is hard, but the rewards make it worthwhile.
I wish you the best of luck, but you don't need that.
Rab