Here’s why your watch says you ran more than a marathon

There’s basically no such thing as running a perfect 26.2 (or 13.1, or 6.2) miles on a certified race course.

watch 2717 miles
Trevor Raab

I was running along the Thames during the London Marathon, looking from my watch to the spires of Westminster, when I saw it: 26.2 miles. Boom. I did it! But wait...where was the finish line? I looked up. I didn’t see it. I found out about 10 minutes later, it was still another mile away.

It took me just under four hours to cross the finish line of the official 26.2-mile course, but my watch said I actually ran 27.2 miles in that same amount of time. WTF?

A race course is an officially measured distance. When a course is being measured, 'official certifiers carefully ride all the tangents of the course [a tangent is the line that touches the inside of a curve] from the start to the finish without runners or cars on the road to measure courses,' explains Dave McGillivray, president of DMSE, Inc., the Race Director at the Boston Marathon since 2001. They do that with a tool called a Jones Oerth counter attached to the front wheel of a bicycle; that counter is then calibrated over a surveyed 1000-foot calibration course to ensure accuracy.

Certified courses, like the major marathons, are measured at the shortest possible route (or SPR), explains Todd Buckingham, Ph.D., an exercise physiologist at the Mary Free Bed Sports Rehabilitation Performance Lab. 'USA Track & Field says "the shortest possible route is a reasonably well-defined and unambiguous route that ensures all runners will run at least the stated race distance."' he says.

'At least' is the key phrase there: In plotting a course, certifiers also build in something called a ‘short course prevention factor’ of 0.10 per cent—meaning the course would be 26.23 miles along the SPR, Buckingham adds. The point: 'to make sure that nobody runs shorter than the race distance,' he explains. 'In order for the course to qualify as a true marathon distance, everyone has to run at least 26.2 miles. And if it were possible for someone to run shorter than that, it wouldn’t be certifiable.'

In theory, you’d ideally want to follow that exact SPR as you run, but that’s hard to do if you’re zig-zagging around thousands of others and navigating dozens of turns. And the more you veer off of it, the more likely you are to run over your anticipated distance. 'It’s almost impossible to run the exact, certified distance of any road race unless the course is a straight shot with no turns and no other runners to run around,' says McGillivray.

That can be a mental mindf*ck, especially in longer races when you’ve been saving your energy for that final push—which is now farther away than you originally thought. Unfortunately, there are no shortcuts on race day. But there are a few ways to tweak your training and mindset so you’re better prepared to run a little farther and/or faster, especially if you’re chasing down a goal time.

Don’t blindly trust your watch!

Surprise: The GPS in your running watch is not perfect. In fact, GPS-enabled watches are guilty of a 'systematic overestimation of distance,' a 2016 paper in the International Journal of Geographical Information Science determined. That’s because GPS watches don’t continuously track movement; instead, they ping your location to a satellite at different intervals and then measure the distance between each ping. If the satellites are slightly off each time they try to pick up your location, those discrepancies (read: extra distances) can add up.

Instead of getting stressed every time your Garmin beeps before the mile marker, turn off the auto-lap feature on your watch—which automatically tracks each mile—and lap your watch manually every time you pass a mile marker, says Ben Rosario, head coach and founder of the Hoka NAZ Elite team in Flagstaff, Arizona. 'That’s going to give you a better idea of your pace and help you run a smarter, smoother race,' he explains. You can practice this in training, too; Rosario’s pros use mile markers he places on a regular route to split their workouts. If your local route doesn’t have mile markers, use a loop or go somewhere familiar where you’ve measured the distance in the past (think: 20 city blocks equals a mile).

Adjust your pacing.

Let’s do the math: If your goal is a sub-3:00 marathon, that means you have to run a 6:52.2 per mile pace for 26.2 miles, says Buckingham. But if you’re running at least an extra tenth of a mile, that adds on 41 extra seconds or an extra 1.6 seconds per mile that you would have to cut off your pace in order to clock that sub-3:00 time. Your real pace would need to be a 6:50.6 per mile pace.

If you know you most likely will be running farther than the actual race distance, you can take that into account when thinking about your race pace. 'The good thing is that you don’t need to run 10 seconds per mile faster than your goal pace,' says Buckingham. 'Instead, aim for one to two seconds per mile. Over the course of a marathon or half marathon, that time savings will add up and help you finish inside your goal time.'

And training to run those faster times will teach your body to recognize how a certain pace feels, which can help you become less of a slave to your watch, says Rosario. 'Looking down at your watch too often uses a lot of mental energy,' he explains. 'You want to get so good at dialing in to your pace that you don’t even need to look at your splits.'

Run the tangents.

This is runners’ jargon for running the SPR. 'Imagine a piece of string that’s stretched along the entire marathon route,' says Buckingham. 'It would be stretched tightly to the edges of the route on each turn and run straight through the curves instead of following along the curves themselves. You want to follow the path of this imaginary string so that you’re not running farther than you have to.' Running the outside of a turn can add up to 40 feet to your route, or almost a half mile of extra mileage over the course of a marathon, according to some estimates.

First, you want to study your course so you know when to expect big turns that could help you shave seconds off your time. In training, practice taking those turns gently, says Rosario. 'If you’re running down the middle of a path and you see a right turn coming up, you don’t just want to immediately go to your right, you want to try to take an angle toward that corner from where you are over the course of 100 or 200 meters,' he explains.

Keep up your momentum as you tackle turns.

Most runners slow down as they navigate turns. Picking up the pace instead can help you shave a few seconds off your time, but know this: 'Frequently changing pace requires more energy than running at the same pace throughout the race' says Buckingham. Surging like this ups your heart rate and zaps more carbs, adds Rosario, which can make you burn out earlier.

Instead, 'stay as even-keeled as possible in terms of pace as you make a turn,' says Rosario. That’s why you’re going to want to cut across over the course of 100 or 200 meters: For one, that’s how that shortest possible route is measured, but sharply cutting across a field of runners can be damn near impossible in a large race unless you slow down—the opposite of your goal here.

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