Keeping a constant race pace

For those of you who run street races from 10 km and up, proper pacing is paramount. If you start too fast, chances are that you run out of fuel early in the race. If you start too slow, you will lose time and end up with a suboptimal finish time. These wisdoms are true for a 10k, but more so for a half marathon, and definitely for a full marathon. Current pace from GPS tends to be utterly imprecise, because you get a GPS waypoint every 5 seconds (5-10 meters) or so, and each of the waypoints has an error of 5-10 meters in all directions. While this is fine if you move fast, say in a car, it is way to imprecise for running. Most people set their watches to display the average pace of the current lap, which isn’t too bad given the inherent rounding errors, but in this case if you change pace (inadvertently or on purpose) it will take a long time until the watch gives you a readout.

Quite some time ago I came across fellrnr’s blog and tried to follow his advice of using a Garmin footpod and calibrate it accordingly. I even bought a Forerunner 610 which is capable of displaying current pace from the footpod instead of GPS, because my Forerunner 620 (that has many other good features over the 610) did not support current pace from the footpod. Garmin never reacted to users’ requests to add this feature in a firmware upgrade – and I am pretty sure their firmware is modular enough to make adding it very easy. I suspect the FR620 was already withdrawn from further development internally at this point in time.

Sadly, I never managed to calibrate my footpod properly. I used the racetrack for example to run 3x 4km (10 laps) at various paces (say, 4:40, 4:30, 4:20/km) and used fellrnr’s calibration tool to determine the corresponding calibration factor. In a subsequent race, it never worked out. GPS pace turned out to be less inaccurate each time. Bummer!

About two years ago I proposed a new idea (to me, at least) to Garmin. How about adjusting the distance on the fly during a run by pressing the lap button when passing a kilometer or mile mark. These marks are usually quite accurate by a couple of meters at most, and the relative error becomes smaller and smaller the longer the race. Alas, they also ignored this proposal. Not that I was too surprised.

Enter the ConnectIQ capable Garmin devices, like the Forerunner 735XT and others. They allow developers to create small apps against a software development kit (SDK) in order to expand the possiblities of a device. About a year ago, Garmin user Peter had apparently a very similar idea, and created a ConnectIQ data field that he called Peter’s Race Pacer

This data field does exactly what I proposed to Garmin back then (and a couple of things more): You press LAP at every kilometer or mile marker (lap distance is configurable), and as soon as you press the button, the current elapsed distance is adjusted on the fly, as well as the current pace (from GPS or footpod, depending on your sensors settings). Even better, you can set a target finish time, and the data field will calculate the perfect pace (PP) from your current position to the finish line depending on your performance so far, and it will also calculate your estimated finish time (ETA) assuming you maintain your current average pace.

The best part as far as I’m concerned is the ahead / behind field which tells you how much time you are ahead of (green) or behind (red) your planned schedule. So if you’re slightly faster than your anticipated average pace for your target finish time, the field will be green, and the ahead time will increase on and on. At each time in the race, you will know exactly where you are. The data field will also record the ahead time to a separate data track in the FIT file, and you can analyze it in the Garmin Connect web portal afterwards (and in Mobile as well).

Plus a couple of more gimmicks – be sure to check the support page.

I tried Peter’s Race Pacer in a 10k and a half marathon in autumn 2017, and was very happy with the result. Plus all subsequent races from then on, including the Frankfurt and Boston marathons. The donation was well spent!

So what I do these days is the following. I use the footpod and set my FR735XT to use current pace from footpod (always) and keep autocalibration on during the training (just for the pair of shoes I am planning to use in the race, nothing else, because calibration factors will vary by shoe!). Over time, this will calibrate the footpod measured pace and distance to match GPS accuracy, which for the FR735XT is usually in the 1-2% range if you have good GPS coverage. No need to calibrate the footpod manually as with fellrnr’s solution mentioned above because in the race, Peter’s Race Pacer will take care of the rest by pressing the LAP button. Best of both worlds! Precise and responsive instant pace display and distance measurement  at the same time.

Make sure to deactivate auto-lap on your watch when doing this. You need to set it to manual, as you would do during a custom training. (Peter says you don’t need to but I do it.)

As an example, here’s how I ran the Salzburg half marathon on May 6th, 2018. I had set the target finish time to 1:34:00 hours which corresponds to an average pace of about 4:27/km. But after 5 or 6 kilometers I noticed that I had plenty of power left, and ran most subsequent kilometer splits below 4:20/km. Hence, the ahead value kept increasing from 6 kms (which corresponds with the water stop at the exit of Hellbrunn castle).

ahead / behind recording after the Salzburg HM

This has a very reassuring effect: for example, you know you are 1:30 minutes ahead of your schedule, and even if you need to insert a pit stop, you will still be ahead of schedule. On the other hand, if you’re behind schedule, you know that may not be going to make your target very early in the race. In any case, no need to do weird math in your head while running.

And eventually in the race, the ahead time was 2:00 minutes, and I thought to myself “hey, wow, you can actually run a 1:32 finish or even better if you keep the pace!”

Interestingly, the ahead / behind time can change a lot during the last 1 or 2 kilometers, for example if you have some horsepower left for a fast finish. In the Salzburg race, the ahead value was 2:28 minutes about 800m from the finish, so that I knew that I could finish at or below 1:31:30 minutes if I managed to keep a slightly faster pace than the previous average. In fact, I averaged the final 1100 meters at 4:13/km, and the ahead value increased even more, and in the end I finished in 1:31:12 hours, way ahead of my anticipated finish time. The ahead value had increased way further without me noticing it (4:13 is 14 seconds below the given average of 4:27, which I failed to understand at this very moment). After all, in a fast finish, you don’t check your watch every 100 meters. (Hey Garmin, how about a virtual (holographic) head-up display? 😉 )

This all boils down to: Never give up! 🙂

(The fun part is that I missed my previous personal record by a mere 3 seconds, mainly because I failed to check the ahead time somewhere during the last 500 meters. But I actually thought I was about to just so manage 1:31:30. I need to think about setting the target finish time to 1:31:09 for the next two half marathons in September. 😉 )

The opposite example was Boston Marathon 2018. I had set 3:29:00 as the finish time, and while I was slightly ahead of schedule despite the weather conditions until about 32 minutes into the race, I fell behind from then on (likely, at the first pit stop), and never managed to catch up. 3:30 – gone. 3:35 – gone. However, I knew I would make it sub-3:45 about a mile from the finish (which is pretty d’oh if you ask me).

In any case, Peter’s Race Pacer is a killer app for me. Highly recommended and well worth the donation!

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