I'm in exactly the same boat. 13 days before my first marathon I've twisted my ankle by falling of a kerb during an easy taper run and tore a couple of ligaments. I've been to a physio a couple of times and have been going through the usual RICE actions and it's getting much better.
Anyway, everybody tells me not to worry about loosing fitness, at this stage the training is done. Just to be careful not to put any weight on so cut back on the food a little if you're not training.
I'm doing the 'contrast bathing' thing at the moment (alternative hot and cold baths for the ankle) every couple of hours and I'm seeing big improvements each day. I'm hoping to be able to at least do a couple of gentle runs before my marathon.
Good luck. I'm sure it won't make a huge difference.