If the rain isn't too heavy and it's in the 40s it will be unpleasant but not terrible if it's not extra windy. Boston in 2015 was similar for me. Showers through the whole race, temps 40s. I wore a light t-shirt, armsleeves, light gloves (they got wet, but it felt colder to take them off than to leave them on), shorts, and a cap with a brim. Honestly once I was wet through, I didn't really notice it anymore, even my feet. Can't do anything about it at that point; I quit letting myself focus on it. I was grateful I remembered the hat with the brim to keep the rain from my eyes and mostly off my face.
Getting into warm and dry clothes as quickly as possible will be crucial when you're done. That year in Boston it was a long time until I finally was able to change and then it was hours and hours before I finally felt deep down warm again, even when dry and bundled in layers upon layers.
I'm running Sunday and not looking forward to any rain but I had success in such before so I'm not going to get too worked up about it this year if it happens again.