Atreyu founder Michael Krajicek uses his "Maff number" and heart rate monitor (almost exclusively) for his marathon training and Ironman Distance Triathlon training/racing. For those of you who haven’t spent the majority of your lives "building this aerobic base," it’s important to understand all aspects of endurance, and the Maffetone Method is an incredibly illuminating resource. BEWARE: It’s often much harder than it seems to go as slow as the Maffetone Method requires :-)
Check out the Maffetone Method for "base building”.
Enjoy the journey!