Keep up the cardio work until you can return to running. Sods law says you postpone by 5 weeks, and then pick up a new injury as you have been marathon training for 23 weeks.
Just be smart about not overdoing the running when you are back, and pick a sensible 'B' target of say 3:15.
I had almost 4 weeks off last time and managed to scrape a pb based on better training and continued improvement. It does feel like a disaster at the time