Osteoarthritis (B): This is the most likely diagnosis given the patient's age and symptoms. Osteoarthritis is a common condition in elderly individuals, characterized by joint pain that worsens with activity and improves with rest. The morning stiffness, although not as prolonged as in rheumatoid arthritis, can still be present. The severe leg pain and nighttime pain are also consistent with osteoarthritis, especially if the joints in the legs are involved.

Osteoporosis (A): While osteoporosis itself might not directly cause severe leg pain, fractures due to osteoporosis could. However, the description provided doesn't directly point towards a fracture. Osteoporosis could be a contributing factor if the pain is due to a fracture, but the clinical picture would typically include a history of trauma or a sudden onset of pain.
Rheumatoid Arthritis (C): Although less likely in a new diagnosis at the age of 70, rheumatoid arthritis (RA) can occur at any age. The symptoms of morning stiffness and pain that worsens with activity could fit RA, but typically, RA would also involve other joints, and the morning stiffness would be more prolonged.

Spinal Stenosis: This condition can cause severe leg pain that worsens with activity, especially walking, and can improve with rest or bending forward. It's a critical diagnosis not to miss because it can significantly impact the patient's quality of life and may require surgical intervention.
Peripheral Artery Disease (PAD): PAD can cause leg pain that occurs with activity and is relieved by rest. It's a condition that could lead to severe consequences if not addressed, including limb loss.
Infection or Osteomyelitis: Although less common, an infection in the bone or joint could present with severe pain, especially at night, and should be considered, especially if there's a history of recent trauma, surgery, or immune compromise.

Paget's Disease of Bone: A condition characterized by an abnormal breakdown and regrowth of bone tissue, leading to deformity and potential for severe pain.
Bone Tumors: Both benign and malignant bone tumors can cause night pain and pain with activity. While rare, they are important to consider in the differential diagnosis to ensure timely and appropriate treatment.