Train specifically. The more closely you simulate the trail terrain you'll be racing on in training, the more prepared you'll be. The more you know the better you can tailor your training to weave in similar terrain and optimally prepare your body and mind for race day.
Merge off road gradually. Although the impact forces while trail running are lower than road running, the demands on your muscles, tendons and joints will be greater when you begin to run on trails. Start your journey to the trails with a few shorter runs during the week and hold this pattern for the first 4-6 weeks. Once you begin to feel comfortable, begin to transition your long endurance runs on the trails.
Watch out for trail drain. One sign you know you've run hard on roads is the unmistakable muscle tightness and fatigue that comes from the impact forces. You can literally feel the effects of the impact on your body. This is not the case on trails. The body hurts less and fatigue shows up in an overall energy drain and decrease in the ability to maintain strong running form (tripping, falling). Like marathon training, it is just as important to follow the flow of easy and hard workouts to allow your body to acclimate and recover efficiently. It is wise to respect the new demands of trail running and in the initial stages treat trail runs as harder workouts until your body adapts. Listen to your body for signs of trail drain.
Modify your long run strategy. Yes, in order to race longer you need to train longer but you don't need to go crazy. Remember to build these long runs gradually just as you did for the marathon training.
Mix it up, run on roads and cross-train. Balance out the rest of your training program with a mid-week 60-80 minute easy run, a faster paced road run (tempo or intervals) to maintain foot speed, and one or two shorter easy paced road runs. Weave in cross-training activities that are lower in impact and will complement the needs of the ultra-athlete. Mountain biking is one of the best forms as you are in and out of the saddle developing core and leg strength in your hips and quads all while training without impact. Don't skimp on the core strength.
Run with the rhythm of the trail. The greatest part of trail running is it teaches you to run by the terrain rather than your watch. Set a goal to run by effort (how you feel - breathing, heart rate) rather than pace. This can and will change the way you run forever.
Be self-contained. Although there will be aid stations on the ultra-course (bananas, chips, sports drink, water, electrolytes and more) you will need to carry fluids and gels with you on the trail. Fueling for an ultra is much different than a marathon because you will be out there longer (due to the longer distance and the demands of the trail). Find the right balance of fuel for you while training this season and learn the hydration system that works for you.
Make friends with walking. Even the best ultra-runners utilize the benefits of walking in training and on race day.
Race like the tortoise, not the hare and be kind to yourself. The secret to successful and joyful ultra-marathon races is in your pacing strategy. Because any given km could be flat, rolling, muddy, technical, it is impossible to race by your watch at a specific pace.