In reply to afshapes:
It really depends where you are with your training. I suspect you could be running again in 4-5 weeks, but that 4-5 would ordinarily be peak training period.
If you are up to 20 miles + on your long run you can probably maintain at least some of your fitness x-training, but you really need to have that mileage in your legs, you cannot make up for it in the last month before the marathon. If you have done the long miles, a couple of long runs before you start to reduce mileage might do it. I would shorten the taper, you will have been resting too much in the weeks prior, but also run some faster races such as 10ks in the few weeks before the marathon.
What would you consider a reasonable time? As it is a first marathon, I would take a few more chances, as it gives you something to aim to beat next time, but you don't want it to be a total soul destroying experience.