Janey - stick with your 8 minute plan for the MK 20, that will give you a good idea of what shape you are in. You definitely don't want to slow down to 9:00/mile pace.
Sometimes you can ask for advice and it doesn't always help or sometimes have a negative effect. Don't let this happen to you. I remember a few years ago people advising to go for a 1:26 half when I was aiming for 1:23. I got 1:23. You have to be ambitious but slightly cautious for a marathon. Go for 8:00/mile for the 20 miler.
Janey, to be honest, now I have had a decent look I think you will be fine. The only two things I would suggest you do, if can, would be to do a medium long run mid week if possible. So that would be about 12 - 16 miles. The other one would be not to go off too fast. Aim for just below 3:30. The problem with a marathon is that it feels really easy until about 18 - 20 miles because of this a lot of people go off too fast and are absolutely wrecked for the last 6 miles. You want to enjoy it.
Just had a quick look Janey. Mine vary as I used a heart rate monitor but I had quite a few runs at 8:10 and faster. If I was you I would stick with 3:30. It's a great achievement for a first marathon. The last thing you want to do is go out too fast and hate it. If you would like to know more just ask.
Hi Janey. I did a 3:29:xx about 6 years ago. I'll have a look at what I did and let you know tomorrow about it. At the moment I think the only thing I would be concerned about at the moment would be your overall mileage. 38 - miles is quite low for a marathon. Any chance you could increase it by 10 - 15 miles a week? Anyway I'll have a look at what I did.