The concept of MAF training or easy aerobic training comes primarily from training elite endurance athletes. It is often misunderstood and misapplied. If we break training intensity into 5 zones (common methodology) we get:
Zone 1: 55-75% max heart rate, easy pace, recovery pace
Zone 2: 75-85% max heart rate, steady pace, marathon race pace
Zone 3: 85-90% max heart rate, moderately hard pace, 1/2 marathon pace
Zone 4: 90-95% max heart rate, hard pace, 10K race pace
Zone 5: 95-100% max heart rate, very hard pace, 5K- 1 mile race pace
The MAF pace is going to be zone 1 or 2 for most people. This is the easy aerobic zone. Training intensity zone distribution for elite endurance athletes tends to be about 80% /20% for Zone 1-2 / Zone 4-5. In other words, they do lots of volume in zone 1-2 with a sprinkling of zone 4-5 work. They do not avoid zone 4-5 completely, even in the off season. The only reason they are minimizing zone 4-5 is to be able to do more volume in zone 1-2. It is a recovery issue. Zone 4-5 work is hard to recover from. Also, the type of physiological adaptations that occur in zone 4-5 tend to plateau after 6-10 weeks, while the adaptations in zone 1-2 don't plateau, even after several years.
The point is, that elite endurance athletes do not slow down to avoid zone 4-5 work. On the contrary, they minimize zone 4-5 work so that they can do more zone 1-2 work. A typical cross country runner might be running up to 120 miles per week!
For the average Joe, who is not doing a high volume of zone 1-2 work, I see no reason to limit intensity to zone 1-2, especially since the American College of Sports Medicine now says that there is a bigger benefit in terms of health and all cause mortality from more intense aerobic exercise.
Those who are not doing a high volume of aerobic exercise (i.e., more than 6 hours per week), would get more health benefits from including some zone 4-5 work. Absolute beginners should do a period of zone 1-2 work first, for a few weeks, before starting zone 4-5 work.
One final point. Fitness comes through progressive overload. If you decrease aerobic exercise intensity, but do not increase volume, you have DECREASED overload. You will detrain. Remember, the reason that elite athletes decrease intensity is so that they can increase volume.
Scand J Med Sci Sports. 2006 Feb;16(1):49-56.
Quantifying training intensity distribution in elite endurance athletes: is there evidence for an "optimal" distribution?
Seiler KS1,
Kjerland GØ
Medicine & Science in Sports & Exercise:
July 2011 - Volume 43 - Issue 7 - pp 1334-1359
doi: 10.1249/MSS.0b013e318213fefb
SPECIAL COMMUNICATIONS: Position Stand
Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory, Musculoskeletal, and Neuromotor Fitness in Apparently Healthy Adults: Guidance for Prescribing Exercise
Garber, Carol Ewing Ph.D., FACSM, (Chair); Blissmer, Bryan Ph.D.; Deschenes, Michael R. PhD, FACSM; Franklin, Barry A. Ph.D., FACSM; Lamonte, Michael J. Ph.D., FACSM; Lee, I-Min M.D., Sc.D., FACSM; Nieman, David C. Ph.D., FACSM; Swain, David P. Ph.D., FACSM