This can be a Vascular Insufficiency, probably Arterial, that is a reduced blood supply to the legs, caused by a decreased caliber of the arteries.
This would cause pain that inreases with exercise.
This can also be from Arthritis of the knees / a neurological cause.
You need to have an examination and consider having a Color Doppler Arterial Ultrasound of your leg arteries, to rule out an arterial cause and also look for other possible causes.
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