Forget about that famous "runner's high." Serious runners will tell you: running hurts, at least at first. If you can't run away from the pain, should you run through it? It all depends on the level of pain.
Your legs have to get used to the hard work of running. Your muscles have to adjust to lactic acid buildup that naturally occurs with strenuous exercise. Your ligaments have to become limber enough to accommodate all the flexing and bending running requires. A little pain is par for the course. A lot, however, is another story. According to the U.S. Consumer Product Safety Commission, more than 165,000 people were treated in hospitals, doctor's offices and emergency rooms for running injuries.
So, how do you know when the pain is normal and when it signals danger? That depends on how bad the pain is, how long it lasts, and how long it takes to recover. Running through pain requires patience, dedication, and a measure of common sense.
If your legs feel only mild pain during a run (say, between 1-3 on a scale of 10, with 10 being unbearable), don't worry. That's probably just your leg muscles complaining, which is normal, especially for new runners. The pain will go away as you run or remain only slightly achy. It's probably safe to keep running. Expect a little soreness for a couple days after running. Once your muscles get stronger and more accustomed to the road, that achiness will subside.
Be sure to stretch after every run, drink lots of water, and take an over-the-counter non-steroidal anti-inflammatory medicine if needed.
Moderate pain (4-6 on the pain scale) can have many causes: poor quality shoes, shin splints, and muscle and ligament strains. This is especially common for new runners, long distance runners, or those that increase speed, distance, duration, or elevation too quickly. If the pain does not subside shortly after you start running, back off a bit. If the pain is severe enough that it causes you to limp or change the way you run, it's time to stop running. You may even need to avoid running for a few days so you don't exacerbate your running injury.
Stretch, use ice, and take ibuprofen. If you notice swelling, put on an ace bandage and elevate your feet. If the injury doesn't get better within a few days, see your doctor. Check in with a sports shoe specialist before you hit the road again, and make sure you have the right shoe.
Severe (more than 7 on the pain scale) or sudden pain signals danger. Be particularly careful if the pain is caused by a fall. This could be a muscle or ligament tear, severe sprain, or even a stress fracture. Stop running immediately, and don't run again until your injury is completely healed and you've gotten the OK from your physician. You may be able to engage in some other type of exercise while you wait for your running injury to heal. Ask your doctor if it's safe for you to swim, bike, or use an elliptical trainer.
Sources:
American Academy of Orthopaedic Surgeons
On Pace to Race
http://www6.aaos.org/news/pemr/releases/release.cfm?releasenum=929
Running Injury Prevention
http://orthoinfo.aaos.org/topic.cfm?topic=A00320
