Our expert says:
Difficult to say, obviously, whtout some kind of examination by a doctor. But often times (not always, mind you), this kind of problem is caused by the movement of the knee not quite being perfect along the groove it sits in. And that is often caused by muscle weakness on one side of the muscles that control the knee - the location of the pain (the inside) seems to suggest this might be the cause. That's not a diagnosis, bear in mind, but a possibility.
So I think that you might get away with the 15km walk, but then you need to probably back off on the walking a little - usually about 50 to 60% of the training is enough to see the problem go away. So maybe do the walk, but then take some time off (especially if it's sore) and then work your way up again.
Then in terms of treating for the future, the best bet might be to see someone. But for some decent exercises to do anyway, try introducing squats into your daily routine - maybe 30 squats a day, and then 20 single-leg squats where you only go down halfway on each side. that helps to strengthen the muscle and hopefully will keep your knee right in the future.
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