Hi Michelle, congratulations! I had a place in the marathon too, but just after I got it I found out I was pregnant myself. I've deferred my place, and even now am glad I did. My advice would be that you pull out, sorry! I know it's very hard to do so after all the work you've done, though. I'm now 29 weeks along and whilst I've had a relatively easy pregnancy the first trimester was the hardest- I had nausea for a month between about 6 and 10 weeks- and whenever I got out for a run I was extra slow because I worried about miscarrying. I think even if you do the marathon you won't get the time you deserve- I did a race a few days before I found out I was pregnant and even though I'd been injured all summer and wasn't expecting a PB, I was recovering and training more, yet half way through the race (just short of 10k) I just ran out of steam and my time was about a minute and a half down on what it might have been.
Luckily as long as you are healthy and your midwife is happy, you can still run (as long as you dont push yourself- my aim is to be able to talk throughout) and I think things have been easier to an extent because I've carried on. I am still running about 30 miles a week over 5 days at a decent pace (about 8.30 average)- but have certainly cut back on the intensity of my non-pregnant training (I have run for North Wales, not now of course tho!). I do need to stop for the loo at least once so the treadmill is very helpful, but I still feel really well and while my bump is relatively small, all my scans are fine! I've done a couple more races whilst being pregnant and although I'm quite a bit slower I'm just enjoying the social side and the atmosphere of being involved. There will always be more races than you could ever want to do, but these 9 months will never be repeated and the most important thing is having a healthy baby at the end!! Good luck xxx