Re: Knee damage
completely agree with zephyr and silentrunner re taking a conservative approach
But do as much exercise as you can - change your exercise, don't stop it.
See a decent physio; do whatever doesn't hurt too much: swim, walk(in good shoes, on the flat) or cycle(low resistance and a long way, not stomping up hills)
Same principles if you use machines to exercise; knees need good muscles, and your head and heart need the exercise
google / sportsinjuryclinic.net or youtube 'static quads exercises', and if you can(pain permitting) try 'lunges' or 'dips' on one knee to strength your medial(inner) quads muscle and re-balance the patella(kneecap) tracking around the femur(thighbone) - caution - this may hurt - if so - physio first
'joint space narrowing' implies that the x-ray-invisible cartilage is worn; this is technically the first stage of 'arthitis' but it is inevitable in all of us and NOT necessarily likely to lead to destructive joint problems.
If your leg and thigh muscles are sound, your technique reasonable, and your training sensible you can be optimistic in the longer term.
a word of caution re MRI - you may end up a VOMIT - 'Victim Of Medical Imaging Technology" - just because you can see something on the picture doesn't mean it is the cause of the problem, or that cutting it out would be a solution...
CoI I'm a slightly cynical GP , and 'carry on regardless' runner
Avatar: snowman on trig point, Bosley Cloud, Feb 09