Muddy obstacle races require whole-body fitness to complete. In order to get the best result possible, you should properly prepare your body for the challenge.
Athletes love obstacle races because they encourage functional fitness. If you can run, manoeuvre your own bodyweight and lift things, you can do anything. What is more, you never worry about your personal best time every race is unique. So take up the challenge and enjoy the race.
Nevertheless, any cyclists, runners or swimmers who want to participate will need to upgrade their exercise in order to develop the total-body fitness required to complete an obstacle race.
The best way to train is to do a run and add in bodyweight drills whenever possible. You need to develop the leg, core, and upper body strength in order to overcome anything on your way.