Achilles tendinitis, tendonopathy, heel pain, all of these can really slow you down and even cause you to stop running altogether.
Eccentric strengthening is the best known exercise to perform for tendon issues. However, you may be missing an important part of this exercise. Eccentrics consist of raising up with both legs and down with only the painful side. Up with both, down with one. But have you included repetitions with the knees BENT as well? You need to in order to address both main muscles that make up the Achilles tendon: the gastrocnemius (crosses the knee) and the soleus (does not cross the knee). It looks funny, but give it a try and tell me how you feel!
As always, let me know if you are having pain or difficulty running and I can help you out. I perform ONLINE physical therapy and ONLINE VIDEO RUNNING GAIT ANALYSIS and can get to you asap. Email me at Kari@RefinedRun.com