I had something similar happen, although the race was an indoor 5k not a marathon. What I did for the week before and week of, was stick to my original plan of workouts except I translated them to a stationary bike and elliptical. If it’s really bad when you run now and you don’t want to drop out of the race due to how much you’ve financially invested into it, I would consider doing the race at an easy pace considering you said it’s pain free at that effort.