Your pace for a half will be much quicker than your pace for a full marathon. It's not just a case of doubling the time it takes for a half to work out how long it will take for a full.
Easy method is to take your hm time, double it and add 20 mins. This assuming similar terrain - ie flat off road for both or undulating trail for both. If you run a flat road hm and want to run a hilly off road marathon then you will need to add at least another 20 mins, possibly more for your marathon time.
That said, from Oct - March you will have more than enough time to train for a marathon.
My only caution being that ramping up your training quickly is the easiest way to come by an injury. esp if you are trying to do all your runs at race pace.
Your long run, or long slow run as it is often known should be slow, that is, at least 1 - 3 mins slower than your marathon pace. (which of course you don't know at the moment). it should be an easy paced run, one which you can happily chat at. This run should be about 1/4 - 1/3 your total weekly mileage, so a 10 mile run your weekly mileage should be between 30 - 40 miles. It doesn't really matter at this stage, just something to think about for the future.
The thing about running slower is that it lets you run further, thus building your endurance without unnecessary fatigue so you will have the energy for the rest of the weeks training. Your faster paced run is what is going to help you improve your race pace.
I would like to ask, without you having yet run a hm, on what are you basing you 'race pace'? Is this the pace that you are doing your long runs at, have you run anyraces yet? I would advise running at least one 10k before you run your hm, just to get used to the race environment if nothing else.
Race pace should be faster than training pace.