Hi.
I would start building up a strong base mileage between now and Christmas. The more miles you do the better. Your long runs should be at a nice slow pace. Don't try to race them or finish off on a sprint.
Running more miles will help your pace, you'll become a stronger runner, greater endurance and you'll slowly be able to up the pace.
We have some very good run leaders at my club, one in particular who will run with anyone who wants to try to go a little faster. He will run with you and push you all the time. Is there anyone like that at your club? Someone who can encourage you to go a little faster even if it's just for a mile or two?
There are so many different ideas out there on how best to train, at our level I think that we can try whatever we want without hurting ourselves too much. Fartlek is just 'speed play' so play. examples maybe - 200m flat out sprint followed by 400m very slow recovery or - 200m 80% effort 200m threshold 200m jog recovery. (The distance doesn't have to be exact).
Threshold training (running at your maximum aerobic steady state)
Any xtraining that helps core strength is good, it could be whatever - core, yoga, pilates, swimming, etc. Whatever is easiest for you. It will help with injury prevention. Yoga and pilates will certainly help with posture which is probably at the route of a lot of running injuries.
You can only do so much before you snap so it has to be what suits you and what you enjoy the most. It's not really a case of one being better than the other or more suited to running.
Right now don't do anything specific for London, just work on getting a sound fitness base. make sure that you are actually enjoying what you are doing. If you don't enjoy it or don't get that feeling of a job well done after a session do something else otherwise you'll just come to resent it. If you don't want to do 2 steady state runs then go off road, add in some variety, run up and down some hills. get muddy, whatever makes you smile. From now until Christmas it's about you getting fitter.
Of course more serious runners will tell you that the only way to run better is to run more and ditch most of the xtraining, or do it as well as, not instead of. real life often doesn't allow for that and most of us need varity to keep our interest.
Come Christmas you'll need to start on your marathon training plan. Take a week out for rest over the Christmas period. A good training plan will help you prepare both mentally and physically for your race.