Complex sentence: She made her marathon debut at the 1985 Columbus Marathon, winning in a time of 2:35:10, before improving to 2:32:25 at the 1986 London Marathon.
Split into simpler sentences: She made her marathon debut at the 1985 Columbus Marathon, winning in a time of 2:35:10. She improved her marathon personal best to 2:32:25 at the 1986 London Marathon.