Your MRI findings suggest you should give up running entirely. The combination of arthritis and a meniscus tear means you're on the slippery slope that may lead to joint replacement. Arthritis in the knee means there are little jagged edges that can shred your meniscus until it's of no value. The past few years have seen studies like the highly regarded "MeTeOR" (meniscus tear in osteoarthritis) study (multi center study at top institutions such as Mayo Clinic, Brigham and Women's Hospital, Hospital for Special Surgery, and 3 or 4 others) examining this issue. The emerging consensus seems to be that meniscus surgery in the presence of arthritis does nothing to alter the course of the problem. Stregthening muscles does not alter the vertical pounding the menicus and joint surfaces take when you run; muscles do not support weight. Take up non-impact forms of exercise and keep the running to an absolute minimum.