LT Workouts
The last key workout for the week is an LT workout. If you have not been doing any higher intensity work, I suggest you begin your threshold workouts at Zone 3 intensity, accumulating some 20 to 30 minutes of Zone 3 time. As you make adaptations, move your intervals to Zone 4 to 5a. More experienced riders can build more time at threshold intensity.
Skills
Be sure to work on your mountain bike skills so you can be relaxed during your ride. You can use one of your weekday training rides to work on bike handling skills. If you're new to the sport, just work on cornering and balance drills in your local park. If you're more experienced, head to the trail and do your skill building work there.
Race-Specific Training
Each race course has specific characteristics. Some courses have long climbs, while others have short rollers. High mountain courses can be rocky with exposed tree roots, other courses can be hard-packed dirt with long sections of deep sand. No matter the characteristics of your goal race, try to ride that type of course in your training whenever possible.
Nutrition Practice
The best time to practice your fueling and hydration strategy is during your long rides. It works best for most people to have a goal range of calories and fluids to consume, per hour, during the race. A good place to start is with one bottle of fluid and 250 to 350 calories per hour.
Taper
As you head into event day, don't forget to reduce your training volume for two or three weeks prior to the race. It is okay, in fact recommended, to keep some intensity in your workouts.
Race Day
On race day, try to keep your ride mostly aerobic. There will be times when your pace drives intensity above the aerobic zone on some of the climbs and technical sections. That's okay; you've conditioned your body to handle this stress and recovery during training. Try to relax and think, "most speed, least cost" while riding.
You can set a reminder chime on your watch to be sure you are consuming enough fuel and fluids during the event. A strong and steady pace wins the race—a personal win.
For more specifics, try these two detailed training plans for 100-mile mountain bike races:
You can also find them in the book Training Plans for Cyclists published by VeloPress.
Put your skills to the test at a mountain biking event.