Simplify the following text for a Grade 6 native English-speaking person.
Original: 50 Kilometers ( 31 miles ) 50 Miles ( 80.5 kilometers ) 100 Kilometers ( 62 miles ) 100 Miles ( 161 kilometers )
Simplified: The most common distances are 50 k , 100 k , 50 m and 100 m , although many races have other distances . The 100 kilometers is an official IAAF world record event .