Rebecca,
I don't think you're alone in feeling scared about what we've taken on. I did 10 miles at the weekend and from the way I felt in the last mile, I was having serious doubts about whether I'd be able to make it all the way round 26 of 'em. But then I realised it wasn't what I could run today that counts, but what I can run on the 23rd April, and there's another couple of months of training to go first.
Like you, I started running about 18 months ago (well, started again, to be pedantic, after a 27 year lay off). I've done a few 5 and 10k's over the last year, and from my PB times for those, I went into FLM training thinking - fantasising would be a better word - I could do a sub-3:30 marathon and maybe aim for a GFA time. Then that changed to 'maybe I could do sub-4:00' and finally to "I hope I can get all the way round in one piece". With that more realistic outlook, I've settled on doing the Hal Higdon Novice program and it's full of sensible and reassuring advice.
The other thing is, whatever time you do you'll be able to tell people you set a PB for the marathon.