Orienteering makes unique physical demands on the body. For those wishing to improve physically in orienteering, nearly any hiking or running training is relevant, but agility, strength, and footing on varied surfaces are important skills to build.
At a young age, the emphasis should be on regular exercise, rather than on intense training. Orienteering, walking, and normal play are good ways to get fit. If you want variety, hiking, biking, and skiing are good too.
For teenagers, training can be running, either on roads or cross country. In the beginning, it's not as important that each training session be perfectly relevant to orienteer so much as that a good fitness base is established. Most training can be similar to that described in numerous running training manuals. Consult such a manual, or an orienteering training guide, and pay attention to the proper age for the training to be done.
Since training is sport-specific, however, some training should take place in the forest, especially at the intermediate and advanced levels, to train balance, agility, and footing on rough surfaces. This training is also useful for injury prevention as it strengthens the muscles required for moving through sand, marshes, and the soft surfaces of woods and fields. Speed is not as important here; until footing is not a problem, training for speed is better performed on a track or smooth surfaces.
Simply running with a map in hand can offer training in map reading on the run. The map can be any orienteering map; route planning of legs on a pre-printed course can be practiced while running.
More advanced training should eventually include training through the various thicker vegetation found in the area. Blueberries, mountain laurel, and roses or other thorny bushes are commonly found in the northeastern states where many "A" events take place. Don't forget to train in rocky areas or on soft ground. Avoid injury by keeping training slower than race pace through difficult terrain until you have adapted to it. Several good books on orienteering training are listed in the Resources section.