This is not an inflammation (‘tendinosis’ is a better term to describe this condition) of the long tendon that connects the calf to the posterior foot, but research suggests that the pain originates from degeneration of or microtrauma to the tendon over several years.
Symptoms are experienced during exercise, but also after periods of rest (e.g. getting out of bed or after driving). Local treatment might be necessary, but part of a physiotherapy assessment should identify those factors that are causing the increased stress on the tendon. For example, running on the toes for prolonged periods, increasing training too rapidly, weakness in the calf muscle producing increased stress on the Achilles tendon or poor hip control that prevents the upper part of the leg generating sufficient forces during walking or running (thus increasing the stress on the Achilles tendon as it tries to compensate for this weakness around the hip).
Treatment aims to reduce the stress on the Achilles tendon by correcting faulty gait or running actions and by strengthening weakened muscles (both in the lower leg and hip/pelvis). Interestingly, as the person’s tendon recovers it is not uncommon to find that their running improves as optimising running will reduce injuries and increase efficiency in running.