Saucony is urging runners to make strides to combat childhood obesity. The Lexington-based running brand’s Run for Good Foundation will roll out a free GPS-enabled iPhone app that allows runners to earn donations for each mile that they run, with the proceeds benefiting youth running programs. The 6-year-old foundation — which recently awarded grants to five nonprofit groups — then will double its $5,000 donation to that month’s featured group when the community monthly goal of 10,000 miles is met.