Muscle Strain or Overuse Injury: This is the most common cause of pain in the back of the knee and upper calf, especially if the pain is exacerbated by walking. It occurs due to strain or overuse of the muscles in the calf and the back of the thigh.

Baker's Cyst: A fluid-filled swelling that causes a bulge and a feeling of tightness behind the knee, which can cause pain when walking. It is often associated with knee arthritis or a knee injury.
Calf Strain or Tear: Direct injury to the calf muscles can cause pain in the upper calf that worsens with walking.
Popliteal Artery Entrapment Syndrome: A condition where the popliteal artery is compressed, leading to pain in the back of the knee and calf during exercise, including walking.
Peroneal Tendinopathy: Inflammation of the tendons that run from the outer lower leg to the foot can cause pain behind the knee and in the upper calf.

Deep Vein Thrombosis (DVT): Although less common, DVT can cause calf pain that worsens with walking. It is a serious condition that requires immediate medical attention to prevent pulmonary embolism.
Popliteal Artery Aneurysm: An aneurysm in the popliteal artery can cause pain behind the knee and in the calf, especially during walking, due to compression or thrombosis.
Osteosarcoma or Other Bone Tumors: Rarely, pain in the back of the knee and upper calf can be a symptom of a bone tumor, which would be a medical emergency.

Chronic Exertional Compartment Syndrome: Increased pressure within the muscle compartments of the lower leg can cause pain during exercise, including walking.
Nerve Entrapment Syndromes: Such as tibial nerve entrapment, can cause pain and other neurological symptoms in the distribution of the nerve.
Vascular Claudication: Pain in the muscles of the lower extremities, including the calf, due to inadequate blood supply, typically occurs during walking and is relieved by rest.