Grade three tear of articular cartilage on medial femoral condyle and running again

Patient: I have a most unusual question. I was a long distance runner for 35 years. I had a Grade 3 tear on the articular cartilage of my medial femoral condyle (right knee). My surgeon thought he was going in to fix a meniscus tear and discovered my meniscus was great but found this other injury instead. I was told I could never run again. It has been three years now and I really, really want to run again. If I start running will I cause a grade 4 tear and further damage? I do not have swelling, nor any pain. Before surgery I mainly had severe swelling, but never real acute pain, just more of it being uncomfortable.I am not an elite athlete, just a mom, who needs to get out and run again as it was something I enjoyed very, very much. Thank you.