Coach Nancy is right. If you don't eat enough while you're training, that's going to cause performance issues. Most people don't realize how important a healthy diet is for an athlete. It's not just about keeping our bodies fit and our weight down, it's about having enough fuel (energy) to be able to run for 26.2 miles.
Have you read any books on nutrition for marathon runners ? Instead of trying to lose weight per se, concentrate on eating for "training". If you eat to diet, that really will hamper performance. It strikes me that if you're eating right and getting enough calories to fuel your training runs, you should lose a little something as you train. Will you lose 20 pounds by May ?
You have your choice, do you want to lose 20 pounds or run the marathon. It's too much to try to do both. Eating healthier by itself will help you lose weight eventually. I'm a firm believer that weight loss is nothing more than a byproduct of a healthy lifestyle. If a person is eating right and getting some regular exercise, the weight will come off on its own.
For now, you might want to check out this book by Nancy Clark. She is a highly regarded and respected sports nutritionist. www.amazon.com/Nancy-Clarks-Food-Guide-Mar