Some great advice on how to run winter trails from "Running Competitor":
Yes, mud can be frustrating, but staying positive will get you through the course in better fashion and likely at a faster pace. Remember, it’s just another variable to hurdle! Roads are predictable. That’s why we love trails, right?
Here's a great winter based training program from Hal Higdon which focus' on time, not speed:
Most significantly, It is a time-based training program, meaning that workouts are prescribed in minutes rather than miles. That recognizes the variability of running conditions due to weather.