Mild Pain: You feel this type of pain when you start to exercise but it usually goes away as you start to warm up and continue running. The pain may be inconsistent and moves around the body, or you feel it bilaterally (in both knees, for example). On a pain scale of 10, it ranges from 1 to 3. Mild pain or discomfort is common and considered safe to run through. Apply R.I.C.E treatment to any areas of concern after your run.
Moderate pain: This type of pain appears as you start exercising, but stays at a tolerable intensity throughout your run. On a pain scale of 10, it ranges from 4 to 6. It rarely passes your pain threshold and it doesn't cause you to limp or alter your running stride. While it OK to finish your run if you're almost done, it's best to listen to your body, take a few days off from running and apply R.I.C.E treatment, allowing your body to heal. A couple of days of rest or pain-free cross training now can save you from a full-blown running injury that sidelines you for a longer period of time.
Severe Pain: Ranging from 7 to 10 on the pain scale, this pain is severe in nature and you can feel it before, during and after exercise. The pain increases as you continue running and will typically cause you to limp. You should never continue running when you feel this type of pain. Consult your doctor and follow his or her recommendations.
Also see: When to See a Doctor for a Running Injury