Fast Starts: How Fast is Too Fast?

More on the Dangers of Starting too Fast in the Marathon

Running with Data
Published in
8 min readOct 25, 2016



  • The faster you start the slower you finish, or so the data says; based on an analysis of Chicago Marathon runners.
  • Starting too fast also dramatically increases the chances that you will hit the wall, at least up to a point; the highest percentage of people who hit the wall go out 20% faster than their average race-pace.
  • Even though going out fast is bad, most runners still do it; less than 15% of runners start conservatively, at or slower than their average race-pace. Fast-starts are the norm.
  • These results are based on an analysis of >400k Chicago Marathon runners from 2005–2016 as part of an ongoing analysis of marathon data.


In an earlier post we looked at the dangers of starting your marathon too quickly. We focused on those runners whose first 5k was their fastest 5k of the race and labeled these people as fast-starters. The findings were fairly stark:

  • Fast-starters had slower finish-times compared to those who did not run the first 5k as their fastest.
  • Fast-starters were also much more likely to hit the wall than those who began their race in a more controlled fashion.
  • And, despite these problems, up to 40% of people were fast-starters.

So starting your race too quickly is not a good idea. But how fast is too fast? If your first 5k is your fastest but only by a small degree is that still a problem? What if it is not your fastest segment but you still go out faster than your average race pace; is that still bad? More generally how does the relative pacing of your first 5k influence your finish-time and likelihood of hitting the wall.

We look at this more fine-grained analysis of starting-pace in this post, once again relying on data from the last 12 years of the Chicago Marathon (2005–2016); that is, more than 400k finisher-records with 5k split times. Chicago is also ideal because it enjoys a fast, flat course so pacing is likely to be closely correlated with effort.

The Relative Pace of the First 5k



Running with Data

Professor of Computer Science at University College Dublin. Focus on AI/ML and data science with applications in e-commerce, media, and health.