Complex sentence: She ran the 800 meters at the 2012 Olympic trials and placed 18th, and also ran at the 2012 Junior World Track meet, where she broke the American high school girl's outdoor record in the 1500m with a 4:11.01.
Split into simpler sentences: She ran the 800 meters at the 2012 Olympic trials and placed 18th. Mary also ran at the 2012 Junior World Track meet, where she shattered the American high school girl's outdoor record in the 1500m with a 4:11.01.