Get yourself properly assessed by a chartered physio / podiatrist, (Your GP can refer you if theres no-one recommended locally, usually the local running club will know someone whos good on biomechanics)it could be foot posture/loading/training technique, referred pain or problems from your back as well e.t.c e.t.c.
FMH's advice to off load the local tissue may well help, but you need to know why that tissue got overloaded to the point where it started to break down in the first place. Turning an acute problem into a chronic one by continuing to do whatever it is that caused this problem isn't going to lead to a world of fun + frolics.
The evidence to back up eccentric treatment of tendonopathies is reasonable + increasingly recognized, BUT the state of the tendon needs careful assessment + it needs to tailored to the individual.