Long slow physiology.

Seems obvious when I write it now, but I guess I'm the sort of person that has to come in at the back door sometimes.

Why running at a pace you can converse is helpful:

The word aerobic literally means "with oxygen" or "in the presence of oxygen." Aerobic exercise is any activity that uses large muscle groups, can be maintained continuously for a long period of time and is rhythmic in nature. Aerobic activity trains the heart, lungs and cardiovascular system to process and deliver oxygen more quickly and efficiently to every part of the body. As the heart muscle becomes stronger and more efficient, a larger amount of blood can be pumped with each stroke. Fewer strokes are then required to rapidly transport oxygen to all parts of the body. An aerobically fit individual can work longer, more vigorously and achieve a quicker recovery at the end of the aerobic session.

Duh.