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Sounds like you did have an incarcerated mensicus. They do sometimes reduce back into place and heal (depending on whether the tear was in a vascular area of the meniscus), or you could have had a 'floating body' that worked itself out of the way of your joint eventually, or was resorbed into your joint fluid. Glad its not bothering you any longer & that glucosamine seems to be helping! Though the research has been iffy, I think its a decent supplement to take, since there's at least a small chance that its helpful. For the same reason, I take Zyflamend myself (natural anti-inflammatory that is based on some strong, pungent spices--turmeric, ginger, etc.)
Polly, in regards to exercise, I do it all: running, strength training, spinning, stair climbing, etc. Since my last surgery in July (hardware removal), I have been thrilled to return to running. I limit myself to twice per week, but with diligent muscle work on my part, my knee is feeling good with it! Meaning: I do often get tight, sore muscles. This is due, in part, I think to having to stop/start my exercise so much due to surgeries, rehab, pregnancies, etc. Now, I feel I'm back on track! I do not have any medial compartment pain. There were at times pain in that general region, but after implementing some deep self-massage techniques (www.julstro.com is a lifesaver!!), I think I was having muscle spasms due to return to exercise-->muscle contraction-->effectual muscle shortening-->pulling on joint-->pain. Anyway, six days a week I'm working out quite hard and doing well!
I agree with you that its best to keep as much articular cartilage around as possible! However, losing a small amount of mensicus will not significantly set you up for arthritis, in my opinion. Losing the entire mensicus would, of course, be more likely to give you some knee pain, over the long haul. Keeping your muscles strong (like you're doing already)
is key to reducing excess strain/pain on the knee joint. Good for you for doing it early. MDs are rather frustrating in their misunderstanding of the importance of physical therapy. I'm a therapist though, so I suppose I have an opinion on this. Of *course* it makes sense to start a patient on PT before surgery, but then again, we in the US just never did quite get that *preventive medicine* thing!! We'd rather fix the problem once it exists rather than preventing it all along. 
You won't be a victim of disuse atrophy if you keep up your muscle work. I think you'll be surprised by how well you do. Ice as much as you can for the first few days (or 1st week) afterward. Keep it elevated while doing your quad sets, and get your full extension as soon as possible. The flexion will come more easily as your swelling decreases.
So glad I was able to ease your mind a bit. As they say, "don't waste your pain." So, I'm glad I was able to share my first-hand knee knowledge with you :) You are a wealth of help regarding MC issues, so thank you! Keep me posted on the knee issue! Oh, also, I noticed that when I'm in an MC flare, my knee is more swollen and painful...just fyi.
I had guessed you were a professional from your very specific and excellent knowledge. I couldn't agree with you more about the medical professions's failure to embrace prevention. Actually, that's why I went into pediatrics - it was more into prevention than almost any other specialty. One of these days this country will wake up and shift its focus to prevention......we can hope, right?
