Is it ok to run to keep fit if i have mild arthritis in one knee ( +partial menec)?

Yes but...: Try, aquatics, body weight, or resistance bands/ tubing biking, and/or elliptical. Aquatics allow you to perform body weight exercises, but being in water actually reduces the stress on your body due to buoyancy. Once you are able to perform body weight activities then look to add light resistance w/ bands/ tubing. This will decrease the "wear and tear" on your joints and inc. Their longevity.

Possibly: Generally speaking if you have lost some of the meniscus cartilage in the knee then you have less of the native cushion/shock absorber thus more of the forces from high impact loads will be transmitted to the cartilage of the end of the bones. Thus you do risk wearing this cartilage down. I usually would recommend low impact aerobic exercise over running: bike/elliptical trainer/swimming.

See your doctor: You could have a host of issues that could cause your knee pain, could be bursitis, tendonitis, arthritis, ligament issues, meniscal injury, sciatica etc. So you may want to see an orthopedic pain specialist that can tell you what is going and help you with the latest treatments or exercise therapies available. You may want to consider stem cell therapy as well. Regenexx.Com