Medial Tibial Stress Syndrome (MTSS): This condition, also known as shin splints, is a common overuse injury in runners, especially after increasing running duration. It presents with pain and tenderness along the medial aspect of the tibia, which matches the symptoms described.

Stress Fracture: A stress fracture, particularly of the tibia, can cause localized pain and tenderness, often worsening with activity. The recent increase in running duration is a risk factor for stress fractures.
Chronic Exertional Compartment Syndrome: This condition involves increased pressure within the muscle compartments of the lower leg, leading to pain, swelling, and sometimes weakness, typically after exercise. The proximal medial lower leg is a common area for symptoms to manifest.
Tibial Periostitis: Inflammation of the periosteum, the layer surrounding the bone, can cause pain and tenderness, similar to MTSS but potentially with more localized symptoms.

Deep Vein Thrombosis (DVT): Although less common, DVT can present with leg pain and swelling. It's crucial to consider DVT due to its potential for severe complications, such as pulmonary embolism, especially in individuals with risk factors like recent long-distance travel or immobilization.
Osteomyelitis: Bone infection can cause localized pain, redness, and swelling. While rare, it's a serious condition that requires prompt diagnosis and treatment to prevent long-term damage.

Tumor (e.g., Osteosarcoma, Ewing's Sarcoma): Bone tumors can cause localized pain and tenderness. These conditions are rare but critical to diagnose early for effective treatment.
Nerve Entrapment (e.g., Tibial Nerve Entrapment): Entrapment of nerves in the lower leg can cause pain, numbness, and tingling. While less common, it should be considered in the differential diagnosis, especially if symptoms persist or worsen despite rest and typical treatments for overuse injuries.