The key requirement for strengthening the aerobic system is stress. Unless you subject your aerobic system to a significantly greater challenge than it faces when you are at rest, it will not adapt. Specifically, you need to elevate your heart rate to at least 60 percent of maximum. So, if your maximum heart rate is 185 beats per minute (BPM), you must run fast enough to get your heart rate up to roughly 111 BPM to boost your aerobic fitness.
