GPS Marathon Pacing

Just thought I'd share this.

Something dawned on me yesterday a few miles into VLM. My target was 3:30, meaning 5.00/km average. According to my GPS, which gives me a split every km, I was slightly ahead of that pace from 3km onward, but after the 5 mile marker (8km), it became obvious that the watch was clocking the kms early, and that I still had some way to go. Not that much to begin with (about 100m after 8km) but it got me wondering how much "extra" distance I would have to cover (according to to watch) and therefore how much faster per km I would have to run to hit 3:30.

In the end I paced it well and got round in 3:28:01, which was an average of 4:54 for 42.51km - so I had an extra 300m to contend with. To run 3:30 for 42.51km I would have needed 4:56/km. Small difference perhaps but had I been bang on the 5:00/km pace I would have been disappointed at the end despite apparently running as fast as I intended to.

Some of this will be deviating from the blue line, some due to GPS inaccuracy - but the lesson learned is that in a long distance race like this, if you're relying on GPS for pacing, you need a bit of a buffer in your target pace as invariably you'll run a bit further than the official distance - I reckon anyway.


  • stutyrstutyr ✭✭✭

    Hi Bokster, I know what you are saying as my Garmin measured 26.3 miles for the marathon.  Also, in London the tall buildings around Canary Wharf & the various tunnels interfere with the signal, giving spurious readings during these sections.

    The way around this is to configure your watch to show average time for lap, and then average pace for current lap.  With auto lap disabled,  you need to manually press the lap button when you pass each mile (or km) marker to give you a more accurate pace reading.  

    This way your average time for lap should reflect your target pace, and the average current pace should be close enough that you know whether you need to speed up or slow down etc.

    I also add the lap counter as a third field as occassionally I'll miss a distance marker (or forget to press the button) - if this happens when you go through the next marker you just press the lap button twice (i.e. the lap count should always match the mile/km marker).

    PS Well done on beating your target 

  • PhilPubPhilPub ✭✭✭

    Simples!: pace band + stopwatch.

    By all means have your Garmin (other GPS units are available) displaying current lap pace, so that can keep an eye on things between mile markers, but don't rely on it for overall pacing.

    Bokster - congrats on your time!

  • literatinliteratin ✭✭✭

    I was even more old-skool as I had pace bands but couldn't read the teeny print on the move. And I wasn't relying on my garmin for anything other than not shooting off too fast. So it was clocks on the mile markers and mental arithmetic for me!

    And well done for beating 3:30 image

  • Thanks all.

    stutyr, good idea re manually lapping to recalibrate each lap. I have 4 fields normally - overall time, overall distance, average pace current lap and average heartrate current lap - which I could probably ditch given how crap the HRM is. 

    Philpub & literatin - what's a pace band?

  • literatinliteratin ✭✭✭

    just a strip of waxed paper with a list of all the mile splits for your target pace on. They were handing them out at the Lucozade and Xempo stands at the expo. You wear it on your wrist.

  • Stop watch on wrist, and the ability (even at may age) to do simple maths in my head whilst running

  • agree Dave, actually I'm one of those who is constantly doing maths as I run on all sorts of things - pace against target, pace against PB, distance to go in miles and kms, number of "segments" (5km segments yesterday), pace per segment etc etc etc. so actually yeserday I felt in full control - but those maths were based on the data from the GPS.

    Even with just a plain old stopwatch, mile markers might be in the wrong places (especially in smaller races) which could see you speeding up and slowing down unnecessarily and that may enough to disrupt your race.

    My point really was that when setting a target pace per km or mile to hit an overall time target, you should give yourself a bit of a buffer since margins can be fine.. or else you could be upset at the end. Hardly revolutionary, just my thoughts.


  • It's been mentioned many times before. People get to the finish and miss their target as the marathon is longer than their gps thinks.

    A simple stopwatch timer and the mile markers work for me. You get 26 checks.
  • I find mallet races are generally more accurate !
  • PhilPubPhilPub ✭✭✭

    Or just enter 10 mile races - makes the maths easier!

    Customisable pace band

  • I used manual lap timing and displayed Average Lap Time yesterday for pacing plus current lap pace for current progress. I also had two pace bands. I was loaded with data! This demonstrates a great willingness to run a controlled race, and can recommend both methods. Excellent information in the hands of a runner who is not going to ignore it (what me?)
  • This isn't new is it?

    Everyone knows that GPS isn't that accurate, and so you need a margin of error if you are aiming for a specific time. It's just common sense.

  • Nose NowtNose Nowt ✭✭✭

    According to my gps, when I did my run last night, I did rather a lot of trespassing - cutting across people's gardens and farmers' fields.  I even ran across someone's roof, and I know that their house is 3 storeys high.

  • Well done RW - I've often fancied a bit of Parkour myself, but don't really think I've the flexibility for it!

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