Pro Penn Marathon Extra Duty Case of 24 Pro Penn Marathon Extra Duty Case of 24 comes with 24 tins in a case. Penn Pro Marathon Extra Duty is Penn’s longest lasting tennis ball featuring exclusive Encore™ Technology for a 22% longer lasting core and high tenacity LongPlay® Felt for extended play. Pro Penn Marathon is the official ball of PTR because of its guaranteed longevity, durability and visibility. Pro Penn Marathon is available in Extra-Duty Felt (for hard courts), Regular-Duty Felt (for soft courts) and Extra-Duty High Altitude (for higher altitudes). Penn’s Professional tennis balls are the most technologically advanced tennis ball providing player with top of the line core and felt technologies that allow the ball to play at its best for longer periods of time.