Achilles tendonitis (AT) can be quite painful, especially if untreated. It's caused by excessive strain on the Achilles tendon, usually by a combination of poor running posture and inadequate heel support. The term itself applies to any of several conditions that cause inflammation of the tendon, none of which will help your running plans.
Treatment can involve corticosteroid injections, but this method is not without its detractors [source: Cantin]. More traditional and less controversial methods include icing and rest, with physical therapy recommended as necessary [source: MedlinePlus].
Like many injuries, you can reduce your chances of getting sidelined with tendonitis by warming up and stretching properly before a run, and avoiding sharp increases in your training. Increase your mileage and running frequency gradually to avoid putting more strain on your lower extremities than they can handle, especially if you plan on doing lots of hard surfaces or run in hilly areas [source: Cantin]. Make sure you're not overpronating (too much inward roll on your feet), since this is another contributing factor to developing AT.