Plantar Fasciitis: This condition is characterized by pain in the heel, which is worse after periods of rest and improves with activity. The patient's description of pain in the heel of the left foot and the exacerbation of pain after being seated aligns with plantar fasciitis. The use of poor-fitting shoes could also contribute to this condition.

Morton's Neuroma: This condition involves a thickening of the tissue around the nerves leading to the toes, causing pain, numbness, and tingling, especially between the toes. The patient's report of discomfort between the digits of the right foot and tingling/numbness in the 2nd digit of the right foot supports this diagnosis.
Metatarsalgia: This condition is characterized by pain and tenderness in the ball of the foot, which could be related to the patient's complaint of discomfort between the digits of the right foot. Poorly fitting shoes and abnormal gait mechanics can contribute to metatarsalgia.
Tarsal Tunnel Syndrome: Similar to carpal tunnel syndrome but affecting the foot, this condition can cause numbness, tingling, and pain in the foot due to compression of the posterior tibial nerve. The patient's symptoms of tingling and numbness could suggest this diagnosis, although it is less common.

Peripheral Artery Disease (PAD): Although less likely given the patient's symptoms, PAD can cause pain in the feet, especially with activity, and is a critical diagnosis not to miss due to its implications for cardiovascular health.
Diabetic Neuropathy: Given the patient's symptoms of numbness and tingling, diabetic neuropathy must be considered, especially if the patient has a history of diabetes or is at risk for developing diabetes.
Infection or Osteomyelitis: Infections of the foot or bone can cause pain, swelling, and tenderness. While the patient's history does not strongly suggest an infectious process, it is crucial to consider, especially if the patient has a history of trauma, diabetes, or immunocompromised status.

Charcot Foot: A condition that affects the nerves and joints in the foot, leading to loss of sensation and structural changes. It is more common in patients with diabetes but can occur in other conditions affecting the nerves.
Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the hands and feet. It could be considered if the patient's symptoms do not fit more common diagnoses, although it is quite rare.
Complex Regional Pain Syndrome (CRPS): A condition characterized by intense burning or aching pain in the hands or feet after an injury or trauma. The patient's complaint of walking on marbles and persistent discomfort could be suggestive, but CRPS is relatively rare and typically follows a distinct traumatic event.