a close friend of mine has been running in marathons, and 10Ks for years now, she just turned 45 and last marathon her knee was killing her, butl, she refused to stop and caused damage to her knee, her doc says she has significant cartilage loss, she’s trying to find out if a person can regrow cartilage and how. Her doc says, “no.” Is he right?
I don’t know if you’d get anything better than ‘listen to the doctor’, but in the interests of giving you a bump, check out this thread which speculates that certain supplements could possibly help, and then this thread that says they’re bad for you. 
thanx! terrific info! i appreciate the bump 