Working to build your aerobic capacity will not make you a stronger and faster runner, but improve your cardiovascular system. Rather than focusing on lowering your heart rate, building aerobic fitness requires increasing your heart rate to an elevated state for an extended period of time without the high impact stress your body gets through running. Cycling, walking, swimming, and rowing are all examples of great ways you can cross-train with the intention of increasing your aerobic fitness. 

Not only will cross-training make you a more effective runner, but it will also give you the opportunity to build your mental strength. Running is a mental game as much as it is physical. Making it mile to mile for distance runners or sprint to sprint for those who prefer shorter distances, your mental strength is what will help you reach those goals. Engaging in any sort of physical exercise helps to  improve mental clarity through the release of endorphins. Depending on whether you want to find calm in the midst of chaos or find release after a stressful day, your cross training is what will get you there. Consider taking a yoga class or doing a meditation to strengthen your mental clarity and feel calm. If you are searching for that stress release, lean into the aggression by trying a higher intensity workout like dance cardio.