I reckon slow runs to burn fat and give more capacity for lean muscle and hydration. "Zone 1" is fat burning, and is around 60-70% of your maximum heart rate. Max heart rate is 220-age. I'm 35 so my max is 185 and my recovery / fat-burning zone is 129 - 143.
Anaerobic energy system kicks in when aerobic energy system is at full capacity (eg for 5K run and up) and is thought to be at 80-90% maximum heart rate, so 157-171. That's about 7:00 - 7:30/mile for me, but I run 200 miles a month.