I have two areas of grade 3+wear on my right knee and wear behind the kneecap. Also some wear in the left knee, all this discovered after three arthroscopies over the past 6 years. After resting from running for nearly a year I started back again very gradually, although I have maintained a lunchtime swim most days for the last two years and cycle whenever possible. Have been running again for over a year now but as fitness returned I have done the predictable-started racing again and overcooked it. So now I am having to lay off running again. I know it is hard but the message is run a little and enjoy it.
Cross train also this will maintain fitness without distressing the knee, and maintain quad strength. I also take glucosamine HCL which I think helps.