A tool for running a casing string assembly into a wellbore includes a degradable plug assembly that temporarily isolates light fluid trapped in a lower portion of the casing string from heavier fluid in the upper portion of the casing string, thereby reducing the horizontal weight of the casing string by an amount sufficient to overcome a drag force. After the casing string is landed at a final location in the wellbore, fluid is introduced to the degradable plug assembly to degrade the plug and clear the axial passageway of the casing string so that tools or other equipment can pass therethrough.