On April 19, 1967, 20 year old Kathrine Switzer started the Boston Marathon, then closed to women, after registering as K.V. Switzer. After 2 miles, the irate race director, Jock Semple, tried to force Switzer off the course. Her boyfriend, Tom Miller, fought Semple off, and Switzer went on to finish. Fifty years after that iconic image helped spark the women’s running revolution, Switzer, 70, will run the race for the first time since 1975 — this year among a field with an estimated 45.5 percent women.