How Hard?
Base training should be relatively low intensity. You're trying to increase your aerobic fitness which improves the heart's ability to get oxygen carried by the blood to the body. The better aerobic fitness you have, the more efficient your body will be. Anaerobic workouts (intense workouts when you're breathing really hard) will also increase your aerobic capacity but won't provide as good a foundation for the season.
Besides, when you're just starting to train, high intensity workouts could cause injury and derail your entire plan.
You can always use a heart rate monitor to know hard your heart is working to deliver oxygen to your body. In general, base training should be done in heart-rate zones 1 to 2.
Side benefits to working at this intensity will teach your body to burn fat during a race instead of using carbs (glycogen). In longer endurance events, this is key.
How Long?
In base training, this doesn't refer to the number of miles you get in. During base training, it's more important to focus on duration; the total time of the workout. This is complicated and depends on your initial fitness level and if you plan on doing short or long adventure races.
If your goal is to do longer (12, 24, expedition) races you must have longer training sessions. If you're doing shorter races you can do either slightly shorter or longer duration training.
More: 12-Hour Adventure Race
When a beginner starts they could range from one to two hours per discipline (run, bike and paddle) each week and total near four to six hours for their first week.
Intermediate athletes should be getting in six to 12 hours per week and advanced athletes should be getting in 12 to 24 hours per week. Some people may not be able to run for an hour, and doing so may cause them pain or injury, error on the side of caution during the first couple weeks.
Because adventure racers are multisport athletes, get this training in on the trail, bike and in the water. In whatever activity you're doing, you're increasing your fitness, so make sure to switch it up and get some time on each.
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Over the base training period you can increase your time as your fitness improves. Four weeks at half and hour on the trail won't be as beneficial during your base period even if you are increasing your distance. It's more effective to increase your duration (your distance will increase too) at the same relatively low intensity.
Don't be scared. Sign up for an adventure race today.