It really depends on what you expect. The nearest I've come to doing a marathon is one leg of the Cork marathon - 5.3 miles - based on a conversation the night before. I jogged bits and walked the rest (did it in 1:09, which wasn't bad at all, I thought). So I'm sure it's possible to finish a full one on reasonably little training provided you're sensible about what you can do - you might take eight hours, but you should get around. Bear in mind too that it's actually really boring; particularly over a longer distance like a marathon, it's a mental as well as a physical test. Also, the slower you are, the less likely you are to get water on the way around.
Aberdonian Stu has done plenty though - which you, shall we say, wouldn't guess from looking at him - and is doing this year's one again, so he'll give you better advice.
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