Generally speaking it takes less training to complete a shorter distance event. If you are aiming for your first event, a 5km or a 10km race could be a good distance to start with. You can, however, run longer events but you should give yourself the appropriate amount of time to prepare. Try following one of our training plans. Also if your lifestyle is such that it will be hard to fit in the training required for a longer event, you may find it more achievable to aim for a shorter distance.