Sorry, folks. Aerobics - as purveyed by gymnasiums and personal trainers - are just part of a panoply of foods, practices, equipment, services, gear, clothing, books, videos and accreditation/certification which conveniently create the impression that you need to pay for fitness.
Watch a healthy child of three or four. Particularly, observe them outside in a park or a forest. See how they run, climb things, stop to look at an ant, run after a butterfly, walk in a stream, clamber over a log, run up a hill. When they come back to you, they are puffing. That's aerobics at its best: natural, exuberant aerobics.
If you can recapture that spirit and translate it into that type of aerobics, you'll be fit.
Contrast it with the treadmill in the gym. A good treadmill can cost as much as a small car and it is designed to set precise running speed, angle of incline, even a pattern of varying belt speeds and incline angles, together with a constantly displayed pulse rate.
They are not called "treadmills" for nothing. The term treadmill brings to mind the epitome of drudgery. Drudgery began with the advent of agriculture and marked the transition to what we now call "civilization". (See Daniel Quinn's books for the full implications of "civilization".) Drudgery intensified with the industrial revolution and reached its apex with Frederick Taylor's scientific management, a well-intentioned breakdown of human actions into mechanical movements. Workers expressed their humanity and rebelled. But the model lives on in the world's gyms, driven by the seductive appeal of the technology and the precision measurements it produces. How would a four year-old react to being asked to do a 30-minute session on a treadmill? How much fitter, both physically and mentally, he or she would be if given free rein and good company in a forest!
There is a place for aerobics, precision measurement, routines and repetitive movements in our fitness-related physical activity. But these should help in diagnosis, prescription and design, they should not drive a physical fitness program. A heart rate monitor can be especially helpful to those who are working to get back in touch with their bodies. Someone who has not exercised vigorously since a visit to a forest at the age of four will need some guidance until they learn what their bodies feel like under different levels of stress. But once they have learnt what rewarding, sustained, stressful activity feels like, they can put the technology to one side.