I think there are two ways of looking at this. On the one hand, the more time you have to prepare, the better. On the other hand, if you're trying to stick to a training plan rigidly, i.e. looking up what sessions it tells you to do on a day to day basis, you may struggle for motivation to be this disciplined over a long period of time.
Personally I would go with something like a 16 - 18 week plan, but work up to it by getting into the general swing of the training pattern. For example, if the plan starts by having you run 5x/wk, 25 miles per week with a long run of 10 miles, start to gradually build up from where you are now, over the course of a few weeks, so that you're more than covering each of these requirements. Get into the habit of running certain key sessions on the same days of the week, e.g. a weekend long run, a week-day semi-long run, one faster session like a tempo run or hilly fartlek, and a couple of easy/recovery runs. Schedule your rest days (or cross-training days) to fit in with the training plan.
An alternative might be to set an internediate goal, e.g. a half marathon around 3/4 months before the target marathon, and follow a separate HM training plan for this, then switch to the full mara plan when the time comes. Chances are, at the peak of a HM plan you would be doing more mileage than necessary to start the mara schedule, but that's OK because you want to periodise your training with occasional cut-backs to give your body a chance to rest and regroup before building up again.
Edited: 18/01/2010 at 09:49