538 did a feature on the stress of being a long distance runner.
Contrary to popular belief, running is a matter of strength training and development, and overtraining can lead to injuries. “Cardio” isn’t about strengtening the lungs. Your lungs are an organ that has no muscles, and will always have the same capacity of delivering oxygen to your body.
What strengtens is the efficiency with which your muscles do a given task on a given dose of oxygen. When you breathe hard, it’s because you have overtaxed your body so badly that it’s starved of oxygen and your lungs must overcompensate to catch up. Your lungs’ efficiency never changes. The amount of work your muscles can do before reaching that hyperventilation point is what changes.
Running every day is like lifting weights the same way everyday. Would you do the latter? Hell no! (You’d at least switch body parts to focus on each day) If you worked out the exact same parts every day you’d see minimal gains and probably get injured. Yet we’re totally fine with running several miles a day, and far more miles a week than our bodies are comfortably capable of handling.
You’re basically overtraining your lower body, and probably running far more than your muscles have the strength to comfortably handle. A lot of runners push their bodies everyday beyond what their muscles are capable of doing on their own… thus their bones and joints are forced to bear more stress than they should, which is how long term injuries, arthritis and other damage happens.
Your bones and joints also have no muscle, and in many cases cannot recuperate and grow the way your muscles can. Any damage you do from excess work stays done.
Injuries are not a mandatory side effect of running. You can do so in moderation, train properly, and avoid them. But most aspiring runners are taught to, literally, run themselves into the ground.
Some people swear by the Couch to 5K starter plan, but I’m partial to Hal Higdon’s approach to learning running. You put in the distance, but you do so at your own pace, even walking or very lightly jogging the distance if you must. You get your body used to the motion of running in a low-stress fashion, and it gradually develops the strength to run at greater speeds.