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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.