The most likely diagnosis is plantar fasciitis. The pain associated with plantar fasciitis is typically sharp in character and present with the first few steps taken after prolonged inactivity, such as upon awakening in the morning or after sitting for an extended period of time. Plantar fasciitis has a peak incidence between the ages of 40 and 60 years. Risk factors include obesity, pes planus, and a sedentary lifestyle. On examination, there is pain with palpation of the medial calcaneal tubercle where the plantar fascia inserts. Pain is also typically elicited with passive dorsiflexion of the toes (Windlass test).

Achilles tendinopathy is typically associated with posterior heel pain, stiffness, and tenderness approximately 2 to 6 cm proximal to the Achilles tendon insertion. Pain usually develops after there is a rapid increase in exercise level. Pain is generally burning, worsens with activity, and improves with rest.

Heel pad syndrome involves localized inflammation of the soft tissues overlying the heel and is often caused by walking barefoot on hard surfaces. It presents with pain in the middle of the heel that is reproducible on examination, and atrophy of the heel fat pad may occur. Neither of these findings is present in this patient.

Stress fractures typically produce pain in individuals who dramatically increase their physical activity or who exercise repetitively with insufficient rest. The pain associated with a stress fracture of the tarsal navicular bone is usually located on the dorsal midfoot with occasional radiation into the medial arch, which is not present in this patient.