In case the OP isn't just trolling, forget about 6-8 hour-long weekly workouts. That's too long and you won't recover to make the most of your week.
Marathon training, with some minor adjustments, works well to prepare for ultra distance events. Weekly:
-one long run 20-30 miles, easy run or progression, on trails if possible;
-one tempo session on flat terrain (run to pace);
-one moderate-to-hard effort on hilly trails;
-everything else is easy/moderate aerobic running, plus some strides.
Hill repeats if you need vert but don't live near mountains, although I think flat tempos translate well to runable climbs; for steeper, mountain courses, strengthen your quads in the weight room. If the race involves hiking, get comfortable with hiking. If there's a lot of quad-banging downhill, isometric squats are your friends.
I don't like back-to-back big days, I think they serve no purpose and compromise the quality of your long run and of recovery. If you run decently high mileage and put the effort you're supposed to in your quality workouts, you'll be running on tired legs anyway.
Use secondary races (sparingly) to get some longer efforts and to practice your nutrition strategy and test your gear.