The average marathon is 42 km long. if the average person can run the marathon with an average velocity of 2.57 m/s, how long will it take him to finish the race

There's no such thing as an "average marathon".  The marathon run is a world athletic sport in which records are kept, and every marathon is the same distance.  The official distance of The Marathon is  42.195 km. 

Time = (distance covered) / (average speed).

Time = (42.195 km) / (2.57 m/s) = 16,418.29 seconds

                                                   =  4hrs 33min 38.29 sec

Note:
With no information given regarding the direction of any part of the
course, the solution must proceed in terms of speed, not velocity.
The question is written as if 'velocity' were a more formal, technical
word for 'speed', but such is not the case.  They are two different
things.

A foot race is a very awkward place to attempt a discussion in terms
of velocity. On a typical marathon course, the starting line and finish line
are in the same place.  Where that is true, the displacement of a runner
who completes the course is zero, and his average velocity is also zero.