Achilles tendinitis causes pain when running and is common in distance runners. An article published in "Sports Health" in 2012 notes that when you run, the force on your Achilles tendon is more than 12-times your body weight. In light of this, it's easy to see why running is typically discouraged with Achilles tendinitis, and why continued running can aggravate the condition.
Time to Heal
Once you have an Achilles tendon injury, it takes a long time for the tendon to heal. Unlike muscles and other structures of the body, tendons don't have a lot of blood vessels to bring them nutrients and oxygen, and this slows the healing process. Since Achilles tendinitis results from overusing the tendon, continued running further stresses the damaged tendon and makes the condition worse. Running can also increase swelling around the tendon, which interferes with healing.
Make It Easy on Yourself
Although it can be frustrating when you can't exercise, resting the tendon is the only way it will heal. Follow your doctor's instructions. She might suggest you use an appliance such as a heel cup, which you wear in your shoe to reduce stress on the tendon. Physical therapy can also be helpful; as the tendon heals, the therapist may introduce gentle stretching exercises or may massage the tendon to help it become more flexible. Don't increase your exercise or return to running until you have the go-ahead from your doctor and therapist, or you could re-injure your Achilles tendon.