STURBRIDGE - It was an early morning start in Sturbridge as the 43rd Pan-Mass Challenge is underway for thousands of riders.The PMC is the single biggest athletic fundraising event in the world, and on Saturday, riders had to deal with some scorching temperatures.So far, the PMC has raised $831 million for the Dana Farber Cancer Institute. This year's goal is $66 million, which would be a new record. The event aims to benefit patients like Mia Lee, a 6-year-old girl who is undergoing treatment for B-cell leukemia.Her mother, Nar Lee, says she was diagnosed in April of last year. "It's been tough....