Aerobic base training is to target the slow-twitch muscle fibers. 

Used for long-lasting, low level force production, extremely long endurance activities and because of its ability to oxidize lactate the slow-twitch fibers are needed to recover between repeated bursts of high intensity intervals or activity. 

Studies have shown that with endurance training an increase in H-LDH enzymes has been found in slow twitch fibers.  This enzyme plays a role in oxidizing lactate.  It’s not unusual for an endurance athlete to have as much as 65 – 80% of aerobic base training annually.  Depending on the distances of your races your ratio of aerobic base training can vary from 50 – 80%. 

Now before you go and start piling up large amounts of volume the slow-twitch muscle fibers are not fatigue proof.  They’re fatigue resistant and you should observe a gradual increase in volume.  As you spend time in your zone 1 you may find your heart rate drops quicker between intervals and a drop in resting heart rate. 

A training zone system is based on three physiological points; aerobic threshold, anaerobic threshold, and VO2 max.  Understanding your response to exercise with physiological testing can help you identify your needs for development.