There are other options besides adventure races which can help you prepare. Your physical preparation must include working out the way you expect to race. Run with a pack Long hikes or fastpacking are an excellent preparation. Swimming is required for almost every event - swimming pace is much less important than being able to swim while managing a full backpack. Adventure racing always teaches you new skills.
Navigation is probably the most critical.
To work on this, contact your local orienteering club. River reading skills are another key element. Instruction is generally available wherever commercial rafting trips are offered. Rope skills required for racing are generally rudimentary - check with climbing schools in your area for instructions. Practice these skills thoroughly.