Running at my MAF pace feels too fast. Why is that? What can I do about it?

You may have been training the MAF way for a while without realizing it, or you may have progressed well because your aerobic system has developed well.

Perceived exertion---the feeling that a workout is difficult---has very little to do with how aerobic or anaerobic a workout is. What it mostly measures is what percentage of your maximum muscle power you're using. On the other hand, the heart rate measures the relative metabolic load on the body---how hard the body is working to keep itself running through the workout. These are two different things: someone can have extremely strong muscles but a relatively small capacity to fuel them for a protracted period of time.

With competitive endurance athletes, the issue is usually the opposite: their aerobic system is so powerful that it can continually supply energy at a very high rate---as high or even higher than the muscle's maximum ability to contract. So the result is that running at a low heart rate feels hard because there is a low relative metabolic cost but an important use of a muscle's total contractile ability.

But there's another possibility: you may have an abnormally low heart rate. This can come from long periods of excess stress or various other imbalances in the body. I've occasionally seen individuals with low levels of vitamin B1 (thiamine), whose absence can cause abnormally low heart rates. In addition, chronic overtraining can result in lower heart rates (more acute overtraining produces abnormally high heart rates).

If you are healthy (and ruled out any such problems) you might consider aerobic intervals--shorter periods of faster paces while still being aerobic (like a fartlek workout). This can be as short as 100-200 meters at a time for a runner, then slow down for about the same (or as long as it takes to feel recovered) - do this for a half hour or so (with a warm up and cool down).

If you are healthy, this is a great indication that you have incredible athletic potential!