With that assumption, I believe that you are fine doing some, or a good chunk of your training on the road bike. I've designed training plans for several riders for both the Leadville 100 Mountain Bike race and other 100-mile races. Several athletes are in situations similar to yours in that trails are not easily accessible from home. These riders have all been very successful on race day.
That written, there are a few key areas I make sure to address in designing the training in the 12 to 16 weeks prior to race day:
- Do at least 50-percent of your long rides on the mountain bike.
- Ride trails similar in profile to the racecourse. If the course has long climbs and you only have access to short climbs, it may mean doing hill repeats with limited recovery.
- If your best training hills are on the road, do your hill training on the road bike.
- Ride the mountain bike with a set-up similar to what you plan for race day. For example, if you plan to ride the race with a loaded pack on your back, practice this in training. Some riders make the mistake of riding with only bottles in training, refilling frequently at park trailheads. Then on race day when they ride with a hydration pack full of fluids, food and clothes they can experience back and shoulder pain that was not present during training. Sadly, it can mean the debilitating end to a race.
- On the mountain bike training days, include some rides with the intensity you plan to ride at on race day.
- The road bike is great for recovery rides and rides that require more structure (like threshold intervals) as well as some of the early long endurance training days. Use this to your advantage.
More: How to Finish a 100-Mile Mountain Bike Race
The short answer is no, I don't believe you need to be riding your mountain bike 100 percent of the time in order to be successful on race day. Many riders are quite successful using a mix of training on the road bike and mountain biking on trails.
Enjoy the rides.
More: 9 Summer Mountain Bike Races
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