Zendaya takes 1/3 * 6 hours = <<1/3*6=2>>2 hours longer to complete the marathon.
To complete the marathon, Zendaya takes 6 hours + 2 hours = <<6+2=8>>8 hours
The combined time the two take to complete the marathon is 8 hours + 6 hours = <<8+6=14>>14 hours
The average number of hours Casey and Zendaya take to complete the marathon is 14 hours / 2 people = <<14/2=7>>7 hours/person.
#### 7