The worst thing to do after a marathon is nothing. You want to bring your muscles back to a normal contract-relax cycle. Perform a concentric regime with 2-3 sets of 20-30 reps in one set to receive burning sensation in your muscles. Another thing is to go back to the basics and work on form technique drills. It is also recommended to take a hot/warm bath with apple cider vinegar and have some miso soup.