In 1 hour, Mary will have traveled 5 miles/hour * 1 hour = <<5*1=5>>5 miles.
In 1 hour, Jimmy will have traveled 4 miles/hour * 1 hour = <<4*1=4>>4 miles.
Since they are running in opposite directions, the distance between them will be 5 miles + 4 miles = <<5+4=9>>9 miles.
#### 9