The women's marathon record is held by a very famous head nodder. I wouldn't worry too much. Having said that,everyone should pay some attention to running form and work on it. The same as improving a golf swing or swimming technique. Try working on one little thing at a time. There are some good videos out there.
Basically you are saying "I've screwed up my training and what do you advise?" Bit late for advice. I would say go and run/walk the marathon and keep your expectations low. Making the finish line any old how is the best you can hope for. Plan your walking breaks and use them early rather than run until you break and HAVE to walk.