The theory is to train all heart rates and start low and slowly introduce faster heart rates as you build stronger mitochondrial paths in the muscle. Arthur Lydiard was doing this long ago with his base-building phase.
This morning I ran the similar loop as yesterday but kept the heart rate (HR) as close to 125 bpm as possible. That was very difficult but I felt great afterwards. It just showed me that the day before I was pushing the pace just a bit. I hope to be up and running 40-50 mile weeks within the next three weeks then if all is well I will try to ramp up the mileage as much as my schedule allows for Feb, March and April.
Total: 43:00 4.6 miles HR= 126 ave.