Bike Training
In the XTERRA, the bicycle course is considered much more technical than a regular triathlon riding course. That's because of all the maneuvers needed to get through the XTERRA ride in a reasonable time with a minimum of downtime (translation: spills).
Your XTERRA bicycle training should have two phases: putting in sufficient riding time to achieve the fitness you will need on race day, and learning how to negotiate the trails under the pressure of race conditions. You can use your regular bicycle and normal triathlon training to get into race shape. Add some intense workouts — hills would be ideal — to get your body used to an elevated heart rate, which is what you will experience when you hit the trail on your mountain bike in the XTERRA. Do trail work at least once a week leading up to the XTERRA.
Skill Set
The key to your XTERRA ride is acquiring the skills to maneuver over rocks and logs and through water. You have a distinct advantage if you are already a mountain biker. You know what it's like out there, and you have learned the tricks, or at least some of them.
An XTERRA racer in training should find someone who can show how it's done in real situations rather than trying to learn in the abstract. If you don't know an experienced trail rider, check with your bike store. A staff member might be available to help or might know someone who could demonstrate the techniques.
If you are lucky, you won't have to travel far to find a trail that can test you on your biggest trail challenges: logs, big rocks, and water. Most of your learning will be by doing, but an essential element of your training will be figuring out how to unweight the front of your mountain bike while continuing to ride.
Streaming Along
To illustrate how important this bit of balancing can be, picture yourself entering a stream or large puddle. If you charge ahead full bore and your front wheel becomes stuck in the mud or the creek bed, what do you think will happen to your forward movement? Yep, you're going to keep going while the bike settles in for a rest. What you want to do is put your weight on the seat of the mountain bike and lift up on the front end.
A good rule of thumb for riders new to the XTERRA is to trust your mountain bike and let it all hang out. In other words: “When in doubt, speed up.”
Logging Time
When approaching a log that you can't go around, you want to lift up the front of your mountain bike. If you can get the front wheel over the log, don't worry about the back wheel. It will follow.
The essential element of your training will be finding a venue for practicing these difficult maneuvers, learning the art of balancing and seated hill climbing. If you have to learn on your own, you will need more time on the trails and less on the roads while you figure out the techniques and become accustomed to a different way of riding.
See It, Do It
Make it part of your planning for your XTERRA to get to the venue at least one day early so that you can ride the course. Getting a look at the course in advance is actually more important than a rest day. The more you know, the more you will enjoy your race.
During the race, don't concern yourself with cadence or time. Your mountain bike won't have a computer anyway, and it would be little more than a distraction if it did. Just ride.

