Posterior Tibial Tendinitis is not a common tendinopathy that usually occurs in athletes such as runners. The symptoms occur either/or just above or below the medial malleolus and there may be some swelling. Typically the cause is when the cumulative load in the tendon exceeds what the tendon can take, usually from not enough time for the tendon to adapt to changes in load. It is typically associated with ‘overpronation’ if the forces of pronation are high.
This video rant from Podiatry Soapbox probably give the best overview of the condition and its treatment.
The management is not that difficult provided a logical sequence of events is followed, yet there are lots of questions in forums (eg here, here and here) with a mixture of the quality of advice being given.