Running and long term health
I have often been told that running is bad for you, as it will 'wear out the body'. This is a dangerous misconception. The human body is not a simple mechanical device like a car. With a car, the more you use it, the quicker it wears out. That's because a car is not alive; it does not heal itself. If you drive a car fast, the car does not adapt and become faster.
The human body is an adaptive system; if you work your body hard, the body becomes temporarily damaged, becomes heals back stronger and faster (supercompensation) [5]. If you do nothing with your body, the body also adapts, becoming able to do less. Anyone who has trained has seen the way their body changes. Anyone who has stopped training has also seen the opposite adaptation.
It is not only the muscles and cardiovascular system that adapt to training. The bones, tendons, ligaments, joint cartilage, hormonal system all adapt as well. The human joints are passively lubricated - if you don't exercise, the surfaces tend to dry out. The cartilage of the joint has no direct blood supply - it obtains what it needs from the fluid in the joint. Without movement of the joint, the cartilage does not get the nutrients it needs.
A Stanford study [3] followed 538 runners for 20 years, starting when they were over 50. Over the 20 years, the runners were half as likely to die (15% compared with 34%). The runners suffered fewer cardiovascular fatalities, as you would expect, but they also suffered less from cancer, neurological disease, infections and other fatalities. The runners did get injured, but on average 16 years later than the non-runners. The gap between the runners and non-runners also got larger as time went on. Other studies have shown no link between running and Osteoarthritis [4, 6].
1 A Cautionary Note
The Stanford study started with runners in their 50s. It is reasonable to assume that a good proportion of those runners had been running for some years, which would tend to self select the healthiest, injury free people. Running can build up joint strength, but cannot compensate for previous traumatic injury. Exercise stress needs to be gradually applied; going from sedentary to trying to run a marathon without training is likely to result in injury. Likewise, starting to run while significantly overweight can stress joints beyond their ability to cope.
2 References
[1] Essentials of strength training and conditioning By Thomas R. Baechle, Roger W. Earle http://books.google.com/books?id=rk3SX8G5Qp0C&lpg=PA107&ots=o4hJq6EbOT&dq=cartilage%20adaptation%20training&pg=PA108#v=onepage&q=&f=false [2]Cartilage injury in the athlete By Raffy Mirzayan http://books.google.com/books?id=dOr37KU3tVsC&lpg=PA19&ots=qFkuJiZ2N4&dq=running%20cartilage%20lubrication&pg=PA19#v=onepage&q=&f=false [3] Running slows the aging clock, Stanford researchers find http://med.stanford.edu/news_releases/2008/august/running.html [4] Long Distance Running and Knee Osteoarthritis: A Prospective Study http://www.ajpm-online.net/article/S0749-3797%2808%2900353-X/abstract [5]Supercompensation http://en.wikipedia.org/wiki/Supercompensation [6]Does Long-Distance Running Cause Osteoarthritis? http://www.jaoa.org/cgi/content/full/106/6/342