In a marathon, the runners typically stop running at the finish line. We don’t see anyone of them deciding to run an extra mile because the finish line is simply where the race ends.
This saying about following the crowd, likely by Francis Phillip Wernig (aka Alan Ashley-Pitt), is similar to the concept of ending a race at the finish line along with the rest of the runners (i.e. the crowd). The saying reminds us that until we step outside of our comfort zone and pave our own path, our path would only go so far.