Plan your long run, but don't be afraid to alter it
A mistake many novice runners training for their first marathon make is that they don't properly prepare and plan their long run. During their long runs, many marathoners will be running for over two hours, so liquids and possibly gel supplements need to be planned in advance. Plus, what you eat before a long run is often different that what you do on your easy days - in terms of what you eat, how much you eat, and how long you wait between the snack and the run. Finally, you have to carve out enough time in your schedule after your run to do the ancillary work necessary to support your training the rest of the week. Put these things together and you have a serious investment of time. So if you're new to running, and you're just starting a training program, make sure you're planning out your long run.
But this time of year you also need to do something else. You should consider running smaller loops - say a six mile loop three times, or a nine mile loop twice, rather than an 18 mile out and back - because of cold weather and slippery roads. The roads are of special consideration because if you're slipping just a tiny bit with each foot strike, you'll likely have sore abductors and adductors during the run. You will likely be able to finish that run, but you might have created an injury during the run. So be willing to alter your run; if your schedule called for 16, but after one eight mile loop you're freezing and you're legs are fatigued from slipping, call it a day and consider getting in a threshold workout on the treadmill three days later.
The bottom line is a lot of planning goes into a long run, yet during the winter, don't be afraid to alter your plan.
*Coach Jay's advice is provided as general training information. Use at your own risk. Always consult with your own heath care provider for questions relating to your specific training and nutrition.