Nutrition is a key part of any 5K training regimen. The proper diet can mean the difference between a successful run and debilitating fatigue, so pay attention to what you're putting in your body. Complex carbohydrates, the kind you get from unprocessed fruits and vegetables, whole grains, and legumes, can give you the necessary energy to run the race. These foods are low on the glycemic index, which means that they'll help keep your glucose levels steady. Consuming foods that are high on the glycemic index will cause your glucose levels to rise sharply -- and plummet just as quickly [source: Oregon State University].