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GG12, I would suggest serious training starts around the beginning of January. However you should have a very good base and be already running 20 - 30+ miles per week.
To ensure boredom doesn't set in between now and then I would set yourself individual goals. Assuming you are starting from scratch I would suggest running a parkrun (5K) then a 10K then a half Marathon.
What level of running are you at??? Have you run any events before???
The actual Marathon training is generally 16-18 weeks long which for London would be around Christmas I think, however its important to have a good base level of fitness and running in your legs when you start. Its never too early to start but take it easy and enjoy your running, and build up your fitness. Also try entering some races too to get you into the mass participation events, thats good for experience ready for the big day. Plenty of time yet though so the main thing is enjoy....
Depends what you mean by training for it. Knocking up a 50-week spreadsheet of ever-increasing volume and ticking each day off as it passes would be pretty soul-destroying, on the other hand you'll want to make sure that you've got a good endurance base for later in the year when you think about ramping up the long runs and training volume in general.
What running are you doing now? It would be a good idea to look at marathon training plans and see what the weekly schedule looks like, and think about getting your routine easily adaptable to it. But I think it's also useful to break down the preparation into cycles/chunks of the year so that you can focus on some interim goals. So a typical approach would be to focus on improving your fitness for 5k/10k races over the Summer, then follow a training plan for a half marathon as your main target for the Autumn (Sept/Oct, lots to choose from). This would be good prep for getting you familiar with marathon-type training, because you'll need to start getting the long run into double figures if you're not used to that already. Then after a little rest, use November / December to build a good endurance base before picking up a marathon schedule (typically 16 - 18 weeks) Dec/Jan.
Also, have a long term view on what mileage you think you should be doing, based on your current levels, marathon target, etc. - e.g. if you think you need to work up to 50mpw it would be good to be in the habit of getting up to at least 25-30mpw over the summer, maybe building to a max of 40mpw for an Autumn half, dropping back a little to recover afterwards before picking up the marathon schedule. (Figures off the top of my head but not totally random!)