I say you should spend 80% of your training on the road and 20% on the trail. I have a couple of reasons why:

- Look at the size of your rings on your road bike, and look at the size of your rings on the mountain bike. It take as lot more power to crank those big rings than it does to turn the mountain bike cranks.  That power will pay you huge dividends when you get deep into your endurance mountain bike race. Put in some big miles on the road and build that engine. When you go to climb with your mountain bike it will seem easy, when you hit the flats and you can pop it into the big ring you'll find you will be pulling away from your competitors.

- You will learn to "spin" on the road and this habit will help you during climbs, during those long flat sections, and allows you to pull more speed from your efficent pedal stroke. Mountain bikers have a bad habit of mashing the pedals for a burst of speed or while climbing.  If your goal is to complete, and compete in enduro races, you need your legs for the whole time.  Inefficent pedaling  and wasted energy will ruin your day.

- One of the problems with riding on the trail all the time is that it is much more difficult to learn pacing, and maintain a steady heart rate zones.  With the hills and trail conditions changing you won't be able to get the most out of your time training. Long endurance days and Interval training on the road will help prepare you for trail.

- One of the other posters mentioned that you should spend a lot of time cross training.  This is a good idea, but with most of our busy lifestyles, bike training will be your best plan. If your are training for a triathlon then running will be good.  But running with poor technique will greatly increase your risk of injury. I race mountain bikes and do triathlons and have learned that cycling will really help your running, but running won't do a lot for your cycling that can't be done on the bike.

Do some core training, stretching, and some upper body work.  A strong core will help ensure the power from your legs gets transfered to your pedals. Stretching regularly will be your best injury prevention tool. The upper body work is important because if you plan to do an off road century, your head feels pretty heavy on your neck and shoulders by the end of the day, your torso does a number on your shoulders, lower back and triceps, and your hands/forearms will feel like they never have before.

Most importantly, if your riding stops being fun, no training plan will be effective. Keep it fun.

Cheers!