I am really sorry to hear about you injury...boo. Being struck by this exact injury earlier this summer, I would be glad to let you know everything about my experience:)! For converience I will break this into sections.
Timeframe: I was diagnosed in early May, and began running at the end of June. I immediately took a week or two off (missed conference for track:( bummer), and put as little weight as possible on it (I used for the boot, but crutches may be even better since the boot does not fully remove weight from your leg). I began running at the end of June, close to six weeks after the initial injury. I began very slowly with small runs, and eased my way back very slowly (I was paranoid when I came back). I ONLY ran on grass where impact was as small as possible. At first it felt a bit sore, but that seemed to dissipate with time (I still feel it a bit after strenuous efforts like long runs, AT's etc...). I also have a nice calcium bump right where it used to hurt (12 weeks later), but I have heard that these can take six months or so to go away.
Cross Training: I opted for aquajogging, and it seemed to keep me in fairly good shape. Initially, I began to aquajog every other day, then gradually built to aquajogging every day (at first this made my leg ache, too). I aquajogged for a month or so, then added the elliptical as a "stepping stone" to introduce my leg to impact again. Even when I began to run again, most of my aerobic work was done in the pool. It took another three weeks before running became my main form of aerobic work. Aquajogging seemed to keep me fairly fit.
Deadline: I don't think you should lose any of the work you have done so far. If you aquajog diligently, I doubt you will lose any fitness (you may get rusty, but pool running & biking will add to your aerobic base). Running will come back fairly quickly when are healed, so I would not worry about it at this point in time. As for planning your marathon, I would not rush it. I know many runners on my team that have been out for 6+ months due to lingering fractures that never fully healed. I believe you primary objective should be getting 100%. After that, you can look at marathons and map out a plan. It is far better to delay a marathon for a few months than to create a long term injury.
Well I guess thats about it! Feel free to look at my blog to check specifics (ascension to volume etc...), but keep in mind that this may be different for every individual. Good luck! You will be back to running in no time so think +! If you have any other questions feel free to ask!