Stress fracture: Given the patient's age and the location of the pain, a stress fracture is a highly plausible diagnosis. The feeling of instability and the fear that the leg might break are common complaints in patients with stress fractures, especially in the elderly population where bone density may be compromised.

Tibial periostitis (shin splints): This condition is characterized by inflammation of the periosteum, a layer of connective tissue that surrounds bones. It could cause pain in the anterior shin and is common in individuals who have recently increased their physical activity.
Osteoarthritis of the knee: Although the pain is described as being just distal to the knee, osteoarthritis could potentially cause referred pain to this area, especially if there is significant degenerative change affecting the knee joint.
Peripheral artery disease (PAD): PAD can cause claudication pain, which might be felt in the shin area, especially upon standing or walking. The feeling of instability might not be directly related but could be a concern in the context of overall lower limb ischemia.

Osteomyelitis or septic arthritis: Although less common, infections such as osteomyelitis (bone infection) or septic arthritis (joint infection) can present with pain and feelings of instability. These conditions are medical emergencies, especially in the elderly, due to the risk of rapid progression and severe complications.
Deep vein thrombosis (DVT): DVT can cause pain and swelling in the leg and is a significant concern due to the risk of pulmonary embolism. The feeling of instability might not be a direct symptom, but any acute leg pain, especially in an elderly patient, warrants consideration of DVT.
Neoplastic processes (bone tumors): Both benign and malignant bone tumors can cause pain and structural instability. While less common, these diagnoses are critical not to miss due to their significant implications for treatment and prognosis.

Chronic exertional compartment syndrome: This condition involves increased pressure within the muscle compartments, leading to pain and potentially feelings of instability during activity. It's more common in younger, athletic individuals but could be considered in an elderly patient with atypical symptoms.
Nerve entrapment syndromes: Conditions like common peroneal nerve compression can cause pain, weakness, and instability in the lower leg. These are less common causes of anterior shin pain but should be considered in the differential diagnosis, especially if other symptoms such as numbness or tingling are present.