This article was originally posted by Ryan at the original HillRunner.com Blogs.
If you rely on your GPS device to determine your race pace and don’t build in a buffer, you will almost certainly fall slightly short of your goal.
Do you run races with a GPS device? Have you noticed that your GPS usually measures the races as longer than the advertised distance? Your last 5K was 3.2 or 3.3 miles, your last 10K was maybe 6.4 miles? Heck, your last marathon was probably at least 26.5 miles, very possibly 27 miles.
Don’t go yell at the race director before you read on. There’s a reason for this.
Proper race course measurement takes the shortest possible route. Because they need to make sure nobody can complete a race in anything short of the advertised distance, they need to measure the shortest possible route.
However, virtually nobody actually runs the shortest possible route. You simply can’t run in the very inner line of every turn, take perfect tangents between all turns and actually turn on a dime at race pace at every hairpin turn. Other runners may get in the way, your momentum at race pace may prevent you from turning sharp enough to take the shortest possible route. Many things can happen.
In addition, GPS devices aren’t perfect. Earlier this year, I covered why they often measure short on tracks. On a race course, they may measure short or long. It’s hard to say and a lot might have to do with the layout of the course.
In the end, expect to run longer than the advertised distance. I would recommend for most people aiming for a goal time and pacing by a GPS device to aim for about 5 seconds per mile faster than goal pace.
This means you also have to train that way. Are you running race pace workouts and making sure you stay on target by using a GPS device? If so, make sure you’re accounting for the faster pace you need to run on race day.