ankle damage + Marathon

hey all, looking for some advice.  I fell off my push bike 3 months ago and badly sprained my ankle.  Despite doing all the right things at the time, it's still causing me pain and i've just found out I've got a place in the London Marathon.  I know it's months away, but I've never run that distance and want to start training again asap.  Does anyone have any idea how I can build up the strength in my ankle?  At the moment, I can't run at all and time is ticking by.  Many thanks