It's possible that it may be too much impact, but it sounds like you have been doing this for a while and 1 minutes runs at 4.5 mph isn't really that heavy as far as impact on joints is concerned. Bearing in mind that I am not a doctor, I would advise this: take a couple days off to let your knees do a little recovering and try some non or less impact exercising like swimming or stationary bike. You might consider changing footwear and/or check how you are walking/running. You may be putting excessive stress on your knees without knowing it. If after a week or so, your knees don't improve, you might want to get a physician's advice, in the event that it may be symptoms of a larger issue. Good luck!
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Keep your body lean, your blood clean and your mind sharp. -Rollins
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