My suggestion is to give it a rest. Do not race on Sunday! You just finished a marathon less than 2 weeks ago. It takes a while for the microfibres inside your leg muscles to heel up. It will keep getting worse if you keep running. You need to rest at least for a couple of weeks. My thoughts.
I had shin splint on the front of my left lower leg once. I was wearing the wrong type of shoes. I switched shoes and it disappeared a week later. But it sure was painful.
Injured or not injured, I think every long distance runner should have a pair of good insoles inside the shoes to support the foot arch in turn to support the spine and to prevent foot and knee injuries.