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" If you can record a time of about 80 minutes, it would certainly be worth aiming for three hours in the marathon."
I think is quite agressive at best. Using RW own predictor (http://www.runnersworld.co.uk/general/rws-race-time-predictor/1681.html), I reckon if you're around 1:25 - 1:26 mark for a half marathon around that time, a sub-3 is realistic.
I was in the same situation as you. My previous marathon PB was 3:30:32 in NYC Marathon 2009, but then over two years I worked on speed, trying to get faster over 5k, 10k and building leg/core strength( was out with knee injury for a while after NY) and once I got my 10K times around 38 mins mark, I decided to have a crack at the marathon distance and got 2:56(that's 34 mins off my marathon PB!!).
During my training, I had only managed a 1:24:20 half marathon time.