Calculating a realistic target race pace

Runners are often asked before a race, “How fast do you plan to run this race?”  The response might be, “Well, my personal record at the 10 kilometer distance is about 53 minutes.  I’m hoping to beat that record in this race.” 

The injury issue and the aging curve

Sure.  Personal records, at any distance, are very rewarding.  But what if you had to take some time off due to injury and you aren’t quite fit enough to beat your record?  Or what if your personal record was set when you were in your early 20s and you will turn 60 before your next race?  Is there a more precise way to determine how fast you should try to run in a race?

Heart race or pace?

I know some athletes who use their heart rate as the guide to how fast they will run in a race.  A few months ago I asked my uncle, who was attempting to qualify for the Boston Marathon, how he would try to pace himself in his marathon.  His response was, “I will try to get into the lower part of my Z3 heart rate zone as soon as I can.  Towards the end of the race I plan to be in the higher part of my Z3 heart rate zone.” 

Racing by heart rate

There are a lot of advantages of using heart rate as a pacing guide in a race rather than relying on pace directly.  To illustrate why, let’s say that you think you can run a marathon at an average 10 minutes per mile pace for the entire 26.2 miles.  Sounds reasonable, but what if miles 5, 6 and 7 of the marathon course are uphill?  Should you stick to your target 10 minute per mile pace?

And what if it’s significantly warmer during the race compared to other races you have run?  Should you slow down from 10 minutes per mile in order to take into account the warmer weather, which makes the body work harder?  How much slower? 

If you use heart rate as your guide, you are relying on how hard your body is working, which is the most important factor in sustaining a pace for the duration of a long distance running event. 

Pace is what it’s about

About a year ago, I began watching videos featuring running coaches Dean Hebert and Joe English.  Hebert and English didn’t place much value on heart rate monitors, either in training or during races.  In one of their videos, Dean Hebert turned to Joe English and asked, “When you crossed the finish line (in your last race) did they measure your heart rate to determine if you would receive an award?”  Well, of course not.  Awards are given to those running at the fastest pace, not those with the lowest or highest heart rates during the race. 

What am I doing?

At some point, I might use heart rate as my guide during races.  But currently, I am using the results of shorter races to calculate my target paces for the longer races. 

Go longer, go slower

There are many different race calculators available online at various running web sites.  You punch in your finish time for a short race, such as a 5 kilometer race, and the web site estimates how long it would take you to finish a 10 kilometer race, a half marathon and a full marathon. 

Built into these race calculators is the assumption that you will run about 4 to 5 percent slower if you double the distance of your race.  So, if you run 10 miles per hour (or 6 minutes per mile) in a 5 kilometer race, a race calculator might predict that you will run 9.6 miles per hour (or 6 minutes and 15 seconds per mile) in a 10 kilometer race.

My own personal race calculator

Last October I ran a 5 mile race in 40 minutes and 49 seconds.  That’s about 7.35 miles per hour or about an 8 minute, 10 second per mile pace.  Four weeks later, in November, I ran a half marathon race in 1 hour, 55 minutes and 33 seconds.  That’s about 6.8 miles per hour or about a 8 minute, 50 second per mile pace.  I ran about 7.4 percent slower in the half marathon than in the 5 mile race. 

Logging the logarithms, base 2

A five mile race and a half marathon aren’t very good math companions when you base your pace calculations on a doubling of the distance.  In order to determine how much a doubling of distance slowed me down, using the 5 mile race and the half marathon race as the raw data, we have to use some logarithms, base 2 logarithms.

2 to the 2.32 power is about 5.  2 to the 3.71 is about 13.1 

This means that the base 2 logarithm of 5 is 2.32 and the base 2 logarithm of 13.1 is 3.71.  The difference between these two base 2 logarithms is about 1.391.  This means that when I jumped from the 5 mile race in October of last year to the half marathon race four weeks later, I doubled my distance 1.391 times.  So, even though I slowed down my average pace by 7.4 percent in the half marathon compared to the 5 miler, I only slowed down my average pace by 5.37 percent for each doubling of distance.  That’s the number I care about going forward.

Why?  It seems, based on those numbers, I slow down a bit more than those online race predictors assume I would.  So, instead of assuming that I will slow down by 4 or 5 percent for each doubling of distance, I will use my personal slow down factor of 5.37 percent. 

A race to predict a race

Next month I will run a 10 kilometer race.  I will likely use the results of this race to determine my target pace for my half marathon in May.  Why not use the 5 kilometer race that I finished about 10 days ago?  After all, it was my fastest time ever at the 5 kilometer distance.  It seems that these slow down factors are more accurate when used with races that are closer in distance.  I don’t think a 5 kilometer race is long enough to be predictive for a half marathon, so I will use the 10 kilometer results instead. 

Despite all of this, once the gun goes off in the half marathon in May, all of this number crunching goes out the window, to some extent.  If the weather is hot, if the large number of participants get in my way, if I am not well rested or if my training didn’t go as well as I had hoped, my prediction might not pan out. 

That’s why you’ve got to execute on race day.  I’ll have potatoes for breakfast that morning, just to make sure my glycogen stores are stuffed full.   



3 thoughts on “Calculating a realistic target race pace

  1. All your math made my head spin! Since I race almost entirely off road, pace means little to me. Every trail is different, and even the same course will differ from season to season. All I have is heart rate and perceived exertion. But road racing requires even pacing, and it sounds like you’ve got a good plan. I suggest running your 10K at your goal pace, and wear your heart rate monitor not for pacing, but to see how your heart rate responds to pace and distance. For me, I train below my MAF of 145 bpm, but a one hour race averages 175-180. For four hour triathlons, I average 165. Knowing this, I use my monitor to keep me out of trouble. Good luck!

    • I have my heart rate data from the half marathon I ran last November. So, I could use that as my guide too. I guess I’m just too “pace focused” to use heart rate. Maybe between now and May 4th, I will change my mind. I guess what holds me back from using heart rate as my guide is that I am afraid, by looking at my heart rate during the race, I’ll think, “Gosh. My heart rate is kind of high. I’d better slow down.” I’ll end up mentally taking myself out of the race.

      Probably and unfounded concern. But that’s really the only thing that has prevented me from using heart rate, instead of pace. That and the fact that I do get inaccurate heart rate results in the first mile or two of a race and also the issue of cardiac drift. But I could work around those issues, if I were serious about using heart rate as my guide.

      I could use pace as my guide for the first two miles and then focus on heart rate. To deal with cardiac drift, I could have one heart rate target for the first half of the race and a slightly higher heart rate target for the second half.

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