The keys to successfully preparing for your first ultra-marathon are:
1. Long Runs. Time on your feet. You need to adapt to spending long periods of time on your feet and moving forward. Longer runs (>4 hrs.) can be broken up with walking breaks. In fact, learning to walk and then run again is a key to success in ultra-marathons.
2. Hills! Hills! Hills! Whether your goal race is hilly or not, the more hills you do in training, the stronger you'll be and the better prepared for your ultra.
3. Middle Distance Workout. The weekend run on weeks you don't do a long run (distance weeks) is very important. If the run is around 15 miles, you'll get an excellent workout, and you'll recover quickly from it. You can run this distance as a tempo run.
4. Your Training Speed Will Be Your Racing Speed. Don't let your long runs drag on for hours. Keep your pace up. When doing your long runs, don't let your pace slow down to a shuffle.