Sorry to hear of your injury. I’m not a medical expert and I assume you have had this diagnosed professionally and are sure of the problem. It really doesn’t matter at this point what caused this injury but the most likely cause is “too much, too soon” as that is the cause of 90% of running injuries. Now you need to focus on getting well. STOP running immediately! If you have a tendon injury you must stop doing what likely caused it and allow ample time to heal. An running focused Physical Therapist or Podiatrist should be consulted for the potential fitment of orthotics. This is because the Posterior Tibial tendon/muscle supports the arch and each time your arch flattens, even a tiny amount, such as in running it stress that tendon.
Therapy for this injury is best done with seated light resistance, high rep exercises of ankle inversion/eversion using rubber bands for resistance. You can find good suggestions here:https://runnersconnect.net/posterior-tibial-tendonitis/
Until your symptoms subside you should not do anything that irritates this condition. If you can hike/walk, get on a treadmill and set the angle up. But don’t run. Be prepared as tendon issues can take months to clear up due to tendons poor blood supply.