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Most often it's iliotibial band syndrome or meniscus inflammation from overuse, especially if you suddenly increased your running load or you're running in bad shoes. Did you recently change your routes or training intensity?
Pain on the inside of your knee isn't always overuse - sometimes it's just that your leg muscles are weak and aren't keeping your biomechanics in check while running.
Check if your knee is caving inward during your push-off - that's the main culprit that's easy to miss if you're only focused on how much you're training. If you switched shoes or started running on asphalt instead of softer surfaces, yeah, that can definitely trigger pain, but even the fanciest new running shoes won't help if your running form is off. Before you start ramping up your mileage, it makes sense to work on strengthening first - planks, glute and quad exercises, and the pain usually goes away in a week or two.
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