Practicing Taekwondo for an hour is not the same as running for an hour. Performing kicks and punches non stop for an hour would be equivalent to running for an hour, but I have never participated in, or even seen, any Taekwondo class, or any other martial arts class, that uses this type of training. There may be some school around that train this way, but they are rare, probably do not have many students, and are probably not profitable, since the schools would not be able to maintain enough students to meet expenses.
Many people can run for an hour, but few, if any, can punch and kick for an hour. Even professional fighters, such as in boxing, the UFC, Pride, etc, use rounds during their fights so they may recover enough to continue fighting. I teach Taekwondo classes in a school where the school owner stresses that students get a “workout” more than he stresses that they learn Taekwondo, so we do a lot of sit-ups, push-ups, jogging, kicking, punching, etc. The emphasis is more on doing a lot more than it is on doing it well, since nowadays, people do not have the patience and discipline it takes to perfect anything—they just want to “get into shape.” If you want your school to remain in business, you have to cater to the wants of the customers. If you are not running a business for profit but are teaching Taekwondo purely for the arts sake, then you may get tough in class. Even with our school’s emphasis on fitness, there is a lot of down time in an hour class. Truth be told, in any hour long Taekwondo class, there are probably 30 minutes of down time where the students are not expending energy. For instance, when doing kicking drills with hand targets, one student holds the target while the other student performs the kicks. Therefore, in a specific length of time, each student is only kicking half the time.
Fitness is a by-product of Taekwondo, not its primary product. Taekwondo is a martial art (a way of fighting with the hands and feet) not a fitness program. If you want to become fit, your best choice is to participate in a fitness program. Only in recent years have the martial arts, Taekwondo included, been marketed as an exercise regime. For centuries, the martial arts only purpose was to make people better fighters; no one cared how fit your were, they only cared if you could kill a lot of the enemy.
Fitness is good to have, but it is not a requirement for a good fighter. Ancient people had to work hard physically all day to just to feed their families; they did no have the energy or time to worry about fitness. They only wanted to be able to take out an enemy before the attacker could harm their families and livelihood.