You can follow a typical marathon training program with a couple of exceptions.
1) Add a little time to your long runs. Consider how long you think it will take you to finish and go from there. However, the time might be skewed, comparatively, for front-of-the-pack and mid-pack runners. It would be beneficial for a 5-hour Pikes marathoner to run a 4-to-5-hour long run in training. It wouldn't be beneficial, in my opinion, for an 8-hour Pikes marathoner to run an 8-hour long run in training. You might want to consider anywhere from 4 to 6 hours for a peak long run (with walking of course) regardless of your expected finishing time.
2) You have to train for strength. Once a week, do body weight leg exercises like air squats, lunges, box jumps, etc. Add a weight vest if you're advanced. Also once a week, try to run a hilly route at a good pace, either tempo run/intervals or fast hiking.
An example week would be Tuesday run an easy run and do leg exercises, Thursday run hills, Saturday long run. If hills are minimal in your area, use the treadmill. I'll go to the gym and run 3-4 miles outside, go inside and hop on a treadmill with the incline at 12% and run 30 min either tempo or 1/1 intervals, and go back outside to do a 3-4 mile cool down.
Apart from strength and more endurance (ability to stay on your feet), it's just like training for a regular marathon.