Millions of people enjoy running for exercise and recreation. It's easy to see why: it's healthy, it's easy to start, and it's non-competitive (unless you race), and it's fun to track your improvement. But, as with anything else, too much of a good thing can be dangerous.
Here are some things to ensure that running will continue to be enjoyable, rather than painful or dangerous to your health.
1. Don't run too much. Experts say that running more than 3 times per week is too much for most people. Obviously, elite athletes are training every day. But they are genetic freaks, and they also have the benefits of the best shoes, special diets, massage therapists, etc. The average person can't match that level of effort without eventually suffering knee, hip, foot, and back problems. If 3x per week isn't causing problems after 6 months of effort, then perhaps add in a 4th day, and see how it goes.
2. Don't run too far. Just like frequency, running too far puts undue strain on the body. Start with modest walks or runs of perhaps 1-2 miles, and stay in your comfort zone. After a few months, you can lengthen your runs or increase your speed. If you can reach 4-5 miles per outing, that's a safe maximum that you can do 3x per week for many, many years.
3. Don't run when it's too hot. It feels great to get out on a warm or hot day, all loosened up in the heat and humidity. But the truth is that it's easy to get quickly overheated. It's better to run in cooler weather, such as early morning or after dinner. Marathoners say that the ideal temperature for them is 40-50 degrees farenheit. Think about that: If you went out for a walk in 40-degree weather, you'd probably want to put on a coat, but marathoners are out there in shorts and a t-shirt. That fact shows how easily our bodies warm up when we exercise.
4. Dress properly. As a corollary to not running when it's hot, think carefully about clothing before a run. Clothing that might feel good for the first few minutes will often generate too much heat for the rest of the run. It's better to start a little under-dressed than to be over-dressed. When I run in cold weather in my neighborhood, I often take off my sweatshirt when I pass a friend's house, and then I pick it up when I return. I get the best of both worlds: warmth at the start and cool as I continue.
5. Get good shoes. It's a cliche to say that "you get what you pay for," but when it comes to running shoes, this is true. It's a huge mistake to buy cheap shoes, or to wear shoes that do not have good cushioning, or to wear shoes that are the wrong size. In the early days, inexpensive shoes might be okay, but by the time you have reached a level of a 3x per week, you need proper footwear. Don't go to your local mass-market running store, either. Go to a place with a staff that includes runners who can help you actually pick the right shoe. It will seem like a lot of money - maybe $90-130 - but it's worth it, if you consider the cost of knee or hip surgery instead.
6. Stay hydrated, but not too hydrated. Especially when you are running in hot weather, it's important to drink water regularly. The key is to drink small amounts of water, even if you are not yet thirsty. If you wait until you are thirsty, you have waited too long. However, researchers have discovered in the last few years that it's dangerous to become over-hydrated, too. The deaths of several amateur marathoners in the last few years have been linked to this phenomenon, rather than heart attacks. So, taking small drinks is the way to go.
7. Pay attention to warning signs. Don't be stubborn or stupid. If you are running and you feel truly ill (rather than just tired), stop running and get help. It's hard to stop when you have a goal in mind, but it's sometimes necessary, especially if you are trying to run an extra-long distance or running in high heat. Watch out for feelings of nausea or dizziness. Be careful if you are super-thirsty and don't have any liquids. If your heart or your left arm (near your heart) starts to ache, stop immediately and call for help.
8. Safety precautions. A few precautions can make the difference. Run with light-colored clothing in the daytime and reflective clothing at night. Don't run on busy streets. Bring water. Bring a cell phone. Tell someone that you've gone out for a run, so that if you don't come back in the expected time, he or she can go look for you.
In conclusion, have fun when you run. It's great exercise that can be done for a lifetime, if you do it carefully.