With other marathons under your belt, general fitness, and a couple of long runs in the bag, I don't see why you can't run this spring marathon. The question I have is what is the goal of the race for you? Finishing comfortably and injury free? Racing to a certain time? Given how your training has gone so far, I would advocate using the race as a way to build your base back up. Train through it, have fun on race day, stay healthy and come up with a plan to attack that big improvement in the fall. Trying to salvage the training you've been able to do so far and piece together a plan to get the improvement now might leave you hurt (either before or after race day) and unable to go for it in the fall. How long had you been running 55 mpw before December?