Congratulations, how exciting! My husband and some friends did it a few months ago - they had a great time. I would have loved to participate in it but my body can be a wreck at times so I opted out.
One thing I know is that you will need awesome GRIP strength; look up Grip Strength exercises online and you will find a ton of info. For example, tie a towel around a dumbbell and lift the dumbbell using the towel as a handle. Also, you'll want to work on your PULL up strength. Naturally, strengthen your calves, legs, and glutes - and be sure to stretch... lots of people's calves cramped up on them while they were doing the course; that could depend on the terrain, though, I'm not really sure.
I'm sure other Mudders here will have great advice for you!
Good luck! You'll do great!