When it's over, give yourself 26 days to recover.
Right away: Get food in you. After a marathon, there is really no cool down. The next day, a walk is good. Or a run if you feel up to it. A walk is probably best for most. A good rule of thumb: For every mile you race, you need a day of recovery. So 26 days until you're back into formal harder training. There are always underlying issues in a marathon, but these don't usually come up until a few days later.