Complex sentence: The Soviet runner was setting a much faster pace than Chataway had ever run and he suffered an attack of stomach cramp which caused him to slow down and as Pirie and Ibbotson came out of the bend they found that Kuts had opened a gap on them.
Split into simpler sentences: The Soviet runner was setting a much faster pace than Chataway had ever run. Chataway suffered an attack of stomach cramp which caused him to slow down, and as Pirie and Ibbotson came out of the bend they found that Kuts had opened a gap on them.