I started running from scratch in Jun01 with the aim of completing FLM 02. In Oct01, with a previous maximum distance of 6miles, I completed the Great South Run (10miles).
Didn't do a great deal of running Oct-Dec then got rejected in ballot and went for golden bond place.
Started loosely following RW training schedules in January (which include a half marathon in around Feb/Mar)and successfully completed FLM in 4.58.01 AND enjoyed it.
I was training three to four days per week, mostly steady runs, no fancy speed work or anything, just 4-5 miles three times per week and one steadily increasing long run.
I read that if you can do 10miles by Christmas you can be ready for FLM in April.
Go for it and enjoy. Hope to see you there.
Oh and by the way, you'll get massive support on this forum which I suspect will get increasingly busy from December onwards!