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Phil - I went from a 3.47 marathon off a 1.41 half, down to a 3.33 off a 1.39 the next year. The following year I went 3.24 off a 1.35 half. So I think you definitely need to keep getting the half times down a bit and the marathon times should then follow.
My mileage would be a bit less than Wardi's - the maximum I have ever done in a week is 62, in a marathon schedule it would tend to average about 50. I'm also 49 (and ran my absolute best marathon of 3.13 at 46, so it's definitely possible to do it in your late 40s!).