In reply to nightmonkeyuk: Take a deep breath. Read everything you can on the subject... then carry on.
I had my third op in 2012 on my right knee and even more cartilidge taken away, The camera showed shocking arthritis too and I've been told the next op will be a knee replacement, probably in the next 5 years. I'm 47. The guy sent a letter to the Doc, outlining no running, no impact sport, no kneeling down (I'm a decorator) and nothing that may risk a heavy landing.
A year later, after changing my running style, I managed to finish a 69 mile ultra despite never having ran more than 10 miles before that last op. No knee pain after too but other parts of me hurt enough to make up for it. I also managed two weeks in the Atlas that culminated in a new 800m route and two weeks in the Alps. I finished last year stronger legged than I've ever been I think.
There's many things I can't/won't do as I know it'll aggravate the knee too much. I have to be careful with drop knees and I wear a brace on the other leg due to the medial ligament being rather stretched due to two separate Grade 1 tears. It's a matter of being sensible, which for me means making sure the muscles around the knee are as strong as possible - hopefully this will mean that they can take more of the strain and I can keep going for longer.
Your surgeon, Doc or consultant will be comparing you to the average member of society who does no exercise and is unwilling to educate themselves well enough to explore different avenues to the goals/lifestyle they want.
Best of luck. IME it's not normally as bad as it seems.