Keir, So you are pure hadding at the moment, I had the feeling that you were building up to a race. No, I think that your content is fine. I think your progression should be to gradually lengthen the 80% portion of your run, to get to 70mins by the time you start marathon training. You can then either:
1. Build that to 83% (and that in essence becomes your MP run) or
2. Add in another 80% run, building the time (from say 40mins) again gradually.
Of course, theres no hard and fast schedule, and you are doing the right elemnts anyway, it would be just tinkering around with the 70,75 and 80% proportions.
Are you going to Hadd right up to the mara, or do a more traditional 12 week build-up?