Jock Itch (Tinea Cruris): This is a common fungal infection in the groin area, often seen in athletes and individuals who engage in activities that involve sweating, such as running. The mild itchiness and location of the rash are consistent with this diagnosis.

Contact Dermatitis: This could be due to an allergic reaction or irritant from clothing, detergents, or other substances that come into contact with the skin, especially in areas like the back and groin where friction and moisture are higher.
Intertrigo: A skin condition characterized by inflammation of skin folds, which can be exacerbated by friction and moisture, common in runners. It can present with a rash and mild itching.
Folliculitis: An inflammation of the hair follicles, often caused by bacterial or fungal infections, which can be more common in areas subject to friction and sweating, such as the back and groin of a runner.

Erythema Migrans (Lyme Disease): Although less common, if the runner spends time outdoors in areas where ticks are prevalent, Lyme disease could be a consideration, especially if the rash has a distinctive "bull's-eye" appearance.
Syphilis (Secondary): This can cause a variety of skin rashes, including ones that are mildly itchy, and can appear in different parts of the body, including the back and groin.

Pityriasis Rosea: A skin condition that starts with a single, large spot (herald patch) followed by an eruption of smaller spots, typically on the trunk, which could include the back. It's often accompanied by mild itching.
Granuloma Gluteale Infantum/Adultorum: A rare condition that can cause reddish-brown plaques in the groin area, more commonly seen in adults with prolonged moisture exposure, such as in diaper areas in infants but can occur in adults, especially in intertriginous areas.