A four-week wait is usually a minimum needed for the Achilles tendon to heal. You are putting your entire body weight on a very small, but powerful tendon. Excessive loading of the tendon from sporting activities is the most common factor in causing further damage to the tendon.
The tendon can rupture from mechanical stress and overuse. Rehab and recovery for that type of injury can take up to a year. Surgery may be needed.
Poor running technique and improper shoes can be important factors in Achilles tendonitis. This is true both for the healing phase and in the case of re-injury. Talk with your coach or athletic trainer about these two factors as they relate to your event.
You may need some changes in your training patterns, too.