Hi Pete,
My main thought is that it's a bit light on mileage, and that the only progression you've got is on the weekly long run. The mid-week runs don't progress at all, and the main problem with this is that the long run will make up an increasingly large proportion of your overall mileage, and I think you will find them tougher than necessary. (They'll be tough anyway!) For example, the week of your last 20 mile run, this run accounts for two-thirds of your total mileage for the whole week.
You will be better off gradually increasing the length of some of your shorter runs. I'd recommend keeping the Sunday runs short, keep the pace very easy, treating them as a recovery run. Your Tuesday run I would gradually increase the length to become a mid-week, 'sort-of' long run, building up to 8-10 miles (10 ideally). Apart from increasing your overall mileage and adding to your endurance fitness, it will support your long run immensely. Imagine yourself starting your training for w/c 5th April - by this time, you will be doing 4 mile runs in your sleep, but dreading the 20 mile run because it's so much longer. Best to have got to the stage where 10 miles feels manageable, and having covered this sort of distance twice a week, you will cover the first half of your long run in relative comfort. It will still be a bit of a slog, but then this is the marathon you're training for!
Thursday's run doesn't want to be too hard because you want to have lots of energy for Saturday's long run, but you could add some distance on, say up to 7/8 miles, and this could be a time to practise running a bit faster, either with a slightly stronger finish, or doing a section of the run at target pace, or a bit quicker. Always have an easy pace warm-up and cool-down though.
Finally, your taper could be a bit more aggressive IMO, i.e. drop the mileage to make sure you're well rested for the race. 18 miles 2 weeks out is a bit heavy. (I'm planning 16 miles, and that's coming down from 75 miles, 6x per week training.) Something like 14 miles will be plenty, that'll still be well over 2 hours on your feet.
Hope that helps.
Edited: 19/01/2010 at 15:46