The MAF idea is that as you become more aerobically conditioned, your running pace / cycling power at that HR improves.
So, rather than running embarrassingly slowly (which seems to be how it starts for many runners), you will be clipping along at an acceptable pace.
When your pace at that HR reaches a plateau, add in some speed work.
Here is an extract from a piece by Mark Allen on his Maffetone journey
"To keep my heart rate below 155 beats/minute, I had to slow my pace down to an 8:15 mile. That's three minutes/mile SLOWER than I had been trying to hit in every single workout I did! My body just couldn't utilize fat for fuel.
So for the next four months I did exclusively aerobic training keeping my heart rate at or below my maximum aerobic heart rate, using the monitor every single workout. And at the end of that period, my pace at the same heart rate of 155 beats/minute had improved by over a minute. And after nearly a year of doing mostly aerobic training, which by the way was much more comfortable and less taxing than the anaerobic style that I was used to, my pace at 155 beats/minute had improved to a blistering 5:20 mile."
More here
https://sites.google.com/view/distance-running-training/articles_1/mark-allen-triathlon