CAPE ELIZABETH — Alexi Pappas solved the mystery of the final arduous stretch of the TD Beach to Beacon 10K. The answer was exhausting, leaving the 25-year-old from Eugene, Oregon, crumpled just past the finish line. It was also worth $5,000 and the prestige of being the first American woman to finish Saturday’s race. “I’m really big on visualization and I felt like it was that last part of the race that was always a mystery to me,” Pappas said.