What happens if at some point of the marathon course I decide I cannot finish?

This will be my first marathon so I'm a bit concerned that I might not make it the whole way round! Is it alright to duck out half way round if I find I'm struggling in the heat?

Apparently there are definitely going to be vehicles on the course to pick up those that feel they must quit. Although they will be roving the course, it will help if you look out for police or volunteers manning the course so they can radio your need for help.