Flying Knickers, I'm in a similar situation myself, although currently planning on going from 5K to 75 miles in 9 months...
I have run a couple of marathons before (last was London 2005, 4:30), but haven't done a lot since then. At the moment I'm at about 5K level, but am currently trying to plan my training to get ready for the Jurassic Coast Challenge (end of March 2008), which is 75 miles over 3 days (1 marathon per day, effectively). Are there training plans for such races, over several days?
My theory is if I can get back to Marathon fitness by the end of the year (does anyone know any good marathons in December?), then build on that through Winter/Spring, then I should be ok.
Not worried about time, though less than 6 hours per leg would give sufficient rest-time.