Peripheral Artery Disease (PAD): This condition is common in older adults and can cause cramping and tenderness in the legs, especially during physical activity. The unilateral presentation could be due to asymmetric disease severity.

Muscle Strain or Overuse: Given the age of the patient, muscle strain or overuse could easily lead to cramping and tenderness, particularly if the patient has recently increased physical activity.
Electrolyte Imbalance: Conditions such as hypokalemia, hypocalcemia, or hypomagnesemia can cause muscle cramping. This could be due to various factors including diet, medications, or underlying diseases.
Diabetic Neuropathy: If the patient has diabetes, neuropathy could cause pain and cramping, although it typically presents with more sensory symptoms like numbness or tingling.

Deep Vein Thrombosis (DVT): Although DVT typically presents unilaterally, it's crucial not to miss this diagnosis due to the risk of pulmonary embolism. The presence of tenderness and swelling would support this diagnosis.
Compartment Syndrome: This is a medical emergency that can cause severe pain, especially on palpation, and can result from trauma or intense physical exertion. It requires immediate intervention to prevent permanent damage.
Aortic Aneurysm or Dissection: While less common, these conditions can cause severe, acute pain and could potentially lead to leg ischemia or other complications.

Amyloidosis: This condition can cause neuropathy and muscle weakness but is much less common and typically presents with systemic symptoms.
Sarcoidosis: Although rare, sarcoidosis can cause muscle inflammation and cramping, among other systemic symptoms.
Peripheral Nerve Tumors: Benign or malignant tumors affecting the peripheral nerves could cause localized pain and cramping but are exceedingly rare.