> Running, swimming, playing soccer and other aerobic exercise do a lot for the cardiovascular system — our heart and blood vessels — but they don’t do much for overall muscle mass or strength.

Preface: like most people here, I'm not a doctor or physiological specialist.

I am decently active, and what I've found from the papers and science that gets brought up, and from my own self experimentation, is that the majority of our aerobic exercise isn't aerobic. In order for your body to use the aerobic energy system, we need a much higher base to raise our lactic thresholds to levels where we can use that system for things like running, swimming, or playing soccer. Instead, our bodies use the glycemic anaerobic systems by default because we don't have that aerobic base.

In order to change this over the winter, I've been doing workouts at much, much lower intensities to try to build up my aerobic ability. For example, this morning I did an hour long walk on a treadmill with slight incline while keeping my heart rate at a consistent 100 bpm. This is a much, much lower level than might even be considered a heart rate for the workout. But I could feel my body doing work the whole time. Other workout is on an indoor bike where I also find a pace where I'm giving effort, but super low intensity to build up my base.

Again, n=1 and I'm not an expert, but my true guess, which is one that I'm working on myself, is that very few people have a good aerobic base to where very little of our exercise is truly aerobic.