The woman bitten by a shark while swimming alone at Rockaway Beach is believed to have lost “approximately 20 pounds of flesh” in the rare attack — believed to be the waterfront’s first since 1953, sources told The Post on Tuesday. The victim, a 65-year-old Queens woman, was attacked off the shore near Beach 59th Street shortly before 6 p.m. Monday — losing a large chunk of her left leg, above the knee, to the shark, according to sources and a photo of her injury. Lifeguards spotted her screaming for help in the water — and pulled her to shore,...