Depends upon your athletic history before the event, your goals after the event and more importantly your metal well being.
Keep off the running until Thursday, even then its 30 minutes light, that's where the most damage is. If you walked most of the marathon then you'll be fine.
But if mentally you're tired, the family has had enough etc then a bigger block of unstructured training will be beneficial.
you may find for 3 or 4 weeks you feel fantastic post IM and really start pushing things again, just to have a real crash. This crash can lead to a CNBA attitude that may take months to shift.
So take it easy, allow yourself to skip a few sessions then pick your next target and get back into it.
Don't wait until January though!