Complex sentence: Due to a shoulder injury, Howard retired from professional tennis only to then compete in long distance running, achieving a 2:38 time in the 26.2 mile marathon and finishing in the top 10% of the Boston Marathon.
Split into simpler sentences: Due to a shoulder injury, Howard retired from professional tennis but soon decided to compete in long distance running. He achieved a 2:38 time in the 26.2 mile marathon and finished in the top 10% of the Boston Marathon.