Brett posted this chart that shows the progression for his MAF (maximum aerobic function) tests. Perfect illustration of what the Maffetone method should do. In 9 weeks he's dropped ~1:45 off his pace per mile at HR 140-150.
And if you're new to the concept he also has a couple of great links to MAFology on his right side bar.
And a quick note. I've been asked a ton of times about running "slow" to get fast. This method does not advocate running slow, there is a minimum HR so you need to run hard enough to keep HR up. LSD doesn't use a minimum and it is possible to run too slow or too easy. MAF advocates running at an intensity that teaches our body to become economical at burning fat. The repetition of this trains our body to become more efficient which then allows us to run faster at the same intensity. This is about building and measuring metabolic and muscular economy. Speed VS HR is nothing more than a measurement of your fitness, so if you have to run slow at a low HR it's because you are not fit. You can't ignore this and simply run faster.
Can you imagine doing this in the weight room? Golly, I'm really weak on bench press and I can only lift 100 pounds... that sucks so I'm going to just ignore my current strength and skip straight to being strong and just put on 200 pounds. Not a great analogy but it has a sharper point.
And in regards to whether an athlete needs to do just MAF training to get better or can you mix in different training like tempo or threshold. It depends entirely on the athlete's fitness. If an athlete comes to me and they test at, lets say 15:00 pace per mile at HR 160 then I see that they are aerobically weak and I do feel that they need to work on correcting this before moving to harder training. This athlete is also going to have a weak structure and they need to run quite easy and build up mileage first. Giving them fast running too soon will increase their risk of injury.
If an athlete comes to me and they are running ~50 miles a week consistently with out injury and their MAF test is closer to their goal marathon pace, then I look at where their weaknesses are and how to approach correcting them. Some athletes should focus 100% on MAF and others can get away with training other systems. It's not all or none.
And weight training is an absolute MUST for a weak athlete. Strength training does far more than just allow you to lift more weight. Along with an increase in strength comes tendon and bone strength for injury resistance. A major increase in calorie burn (I have a 15:00 strength routine that can burn 700 calories. 15:00 of running burns about 150 calories.) Increased fatigue resistance. Stronger muscles increase joint stability. A stronger muscle also carries more oxygen. And having the upper body of a 10 year old girl is neither attractive nor functional. And for those of us over the age of 35, we are no longer maintaining muscle mass or bone density and it will only get harder the older we get to stay strong. Have you ever seen a 60 year old struggle to get out of a car or a chair? That is a lack of strength and can (in most cases) be cured with weights. So even if strength training does hurt MAF progression, the trade off is well worth it.