Though hard surfaces give a great push for you to run faster, you'll probably be able to run for longer periods of time and even go farther on soft surfaces. Some experts claim to get up to 25 percent more mileage on softer surfaces like dirt [source: Glover].
Perhaps the most important reason to train on soft surfaces, however, is that it it puts less strain on your body, reducing the risk of injury. Keep in mind that out of the three events in the triathlon, running is the hardest on your body. Hard surface running is tough on your joints and shins and can lead to stress fractures. Soft surface running, on the other hand, will improve your lower leg strength to help avoid shin splints and ankle sprains [source: Russ]. It's also good for stabilizing torso muscles [source: Bean].
Some soft surfaces can offer runners a more well-rounded workout, because you'll be forced to shift and adjust to uneven terrain, using muscles you might not on flat concrete. Even though dirt and grass come with unexpected rocks and crannies, your body learns to adjust to rugged terrain, which in turn helps strengthen ankles [source: Bean].
Conversely, perfectly flat surfaces overwork certain muscles and underwork others. If you run on a track, for instance, you should at least reverse directions occasionally to avoid repetitive stress on the outside leg [source: Noakes].
However, because your triathlon might involve hard surface running, you don't want to stop training on hard surfaces altogether. Overall, it's best to vary the kinds of surfaces you train on. Incorporate several different kinds of surfaces in your training routine.
Beware of drastic changes in surface, though, which can lead to pain and injury, especially if you aren't already in excellent shape. For this reason, many experts recommend slowly transitioning to a new kind of surface if you aren't used to it: Take only short runs every other day on a new surface before you incorporate it into a daily routine [source: Glover].
Varying your surfaces will also help you avoid the monotony that comes with running on just a track or treadmill. It may sound frivolous, but monotony can discourage many from continuing training.
So fight boredom, give your joints a break and hit the trail.