If masters track had a Mount Rushmore, it would includes David Pain, Al Sheahen and Rex Harvey. On Sunday — two weeks after enduring the flu many caught at the USATF annual meeting in Reno — Rex died of a heart attack in Prescott, Arizona, where he was recovering from appendix cancer. He was 73. I don’t know if the flu weakened Rex, but on Dec. 12 he wrote me: “Today I ate for the first time in 5 days but still feeling woozy around the edges.” When I learned of Rex’s passing via Facebook, my heart sank 1,000 miles….