These feeling are a result of nervous energy. If you let them get on top of you they can rule you and potentially lead to a poor performance. However, if you can turn them into a positive, it can enhance your performance. You can do this by banishing the negative comments from the internal dialogue going on in your head and replacing it with some positive self talk. In a recent study (source) the effects of positive self talk has shown to significantly reduces perceived exertion and enhances endurance performance.
This is the time I ask athletes to leaf back through their training diary and see all the training they have completed, prep races run and, splits achieved. You can get so much positive reinforcement from your training diary, so much so, it is an essential tool in your marathon success.
If your legs are feeling tired and heavy, this can be simply your body adjusting from a higher mileage back to a lower mileage as you taper. This is why it is good to keep a bit of snap in them with some race pace intensity, even if it’s just some strides. The marathon is a long and hard race but getting to the start line fit and healthy is half the battle. If you have done the miles (which is the cornerstone of marathon training) and prepared well, be confident in your ability and go after it. If negative thoughts creep in, banish them and tell yourself your strong and able.
Best of luck to you on your journey and I hope you all achieve your own personal goals.