Aerobic conditioning is a process whereby one trains the heart

Heart

The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 and lungs to pump blood

Blood

Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....

 more efficiently, allowing more oxygen

Oxygen

Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 to get to muscle

Muscle

Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s and organs

Organ (anatomy)

In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

.

Aerobic

Aerobic exercise

Aerobic exercise is physical exercise of relatively low intensity that depends primarily on the aerobic energy-generating process. Aerobic literally means "living in air", and refers to the use of oxygen to adequately meet energy demands during exercise via aerobic metabolism...

 conditioning is a determining factor in performance in events with a duration greater than 2mins. On the athletics track this would relate to all events extending from the 800m.

Usually this is done through cardiovascular exercise, like running

Running

Running is a means of terrestrial locomotion allowing humans and other animals to move rapidly on foot. It is simply defined in athletics terms as a gait in which at regular points during the running cycle both feet are off the ground...

, swimming, aerobics

Aerobics

Aerobics is a form of physical exercise that combines rhythmic aerobic exercise with stretching and strength training routines with the goal of improving all elements of fitness...

, etc. A stronger heart does not pump blood faster but more thoroughly. Trained endurance athletes can have a heartbeat as low as the reported 28 beats per minute in people such as Miguel Indurain

Miguel Indurain

Miguel Ángel Indurain Larraya is a retired Spanish road racing cyclist. He won five consecutive Tour de Frances from 1991 and 1995, the first to do so, and the fourth athlete to win five times. He won the Giro d'Italia twice, becoming one of only seven people in history to achieve the Giro Tour...

 or 32 beats per minute of Lance Armstrong

Lance Armstrong

Lance Edward Armstrong is an American former professional road racing cyclist who won the Tour de France a record seven consecutive times, after having survived testicular cancer. He is also the founder and chairman of the Lance Armstrong Foundation for cancer research and support...

, both of whom were professional cyclists at the highest level.

Although exercising at lower intensities will improve aerobic conditioning, the most rapid gains are made when exercising close to an individual's anaerobic threshold. This is the intensity at which the heart and lungs can no longer provide to the demands of the working muscles and an oxygen debt begins to accrue or when the exercise moves from being aerobic to anaerobic. Aerobic training intensity for most individuals will be <85-92% of maximum heart rate.