In reply to fiendoidel:
marathon's are sometimes described as a race of two halves - a twenty mile half based on training and stamina, followed by a six mile half, driven by determination.
Yes, you could probably do it, based on three runs a week - a speedy 30 mins, an eight miler and then a long run (typically) at the weekend.
To reduce injury you should only add a mile or two a week, so if you're starting from a baseline of 13 miles (22km), then in five weeks you should be able to get up to 18 miles, which you could then sustain for a further couple of weeks. As a rule of thumb, you can usually run 1.5x your training distance, so by getting your legs use to 18 miles, and your body use to the nutritional requirements at that distance, you should get by.
Can't guarantee it'll be injury free though!