Unless you've been running regularly for quite some time, running a 5K race isn't something you can comfortably do without some buildup. Your training schedule should include a steady increase in both speed and distance. This is essential not only to your body, but also your mind; trying to do too much, too soon, will leave you exhausted and discouraged. Even if you think you can do more, it's important not to push too hard. The longer your body has to work up to the goal, the less chance you'll have of injuring yourself. As your strength and endurance increase, you'll be able to run farther and faster.