How wrong can GPS tracking be?

With only four weeks to go before I attempt my first ever half marathon, I am keen to work out how I am going to pace the race.  Even though I lay on the bed this morning, wishing I could stay cuddled up with Keith and Little J, I managed to get out into the open for my 10 mile run.  I never know how I’m going to feel when I step out of the door.  Most of the time, I feel like I’m running through treacle.  For some reason this morning felt different.  I tried my hardest to go out slowly – failed at that one – but amazingly, I felt OK.

Even more amazingly, I continued to feel OK.  Through my normal “that’s it, I can’t do this” kilometres, I felt strong.  Woohoo!

Anyway it got me thinking.  In four weeks time, I should be at the same point in my hormone cycle, so should technically feel the same.  Therefore this would be a good race to see which pacer I should aim to follow.  I was determined not to push myself for this exact reason and see what the results were at the end.

I use two devices to track my runs.  MapMyRun on my phone and my FitBit Surge.  Both devices are started within a few seconds of each other.  Both are worn on my left arm.  Both are pretty much uncovered.  So why then are the results so different?

First 9 km splits

The left picture shows the splits from my FitBit and the right MapMyRun.

8 - 16km splits

They differ from 1 second to 40 seconds!  How on earth do I know which one is correct?  How can I possibly know whether to use the 2:30 pacer or try and push myself to follow the 2:15 pacer?  Aghhhhh

Keith says I should run my own race – but I feel that is too far out of my comfort zone.  I know I slow down and speed up (even though it doesn’t feel like it to me), but I should try and stay consistent – shouldn’t I?

Oh I’m more nervous than ever.

Then there is the issue of water and fuel.  Today I managed both well.  With my water belt on, I started sipping from around 3km and nibbling at 6km.  I honestly felt the difference.  I did 10km on Thursday on a fairly warm day, without water and seriously struggled.  What if I can’t get to the water stops, or there is none left?  Should I just wear my water belt?

How come Mo Farah doesn’t have issues like this?  He ran Great North Run today in 59 minute – 59 minutes!!  I can’t run 10km in 59 minutes.  Agggghhhh it’s too much…..

…..right I’m calm again.

This race is for Andrew, Sally, Skye, Jesse and Baby Hall.  This race is for all those families who have lost their child through a brain tumour or the side effects of the treatment.  This race is for my grandad.  This race is for my wonderfully supportive husband, who goes out of his way to make sure I pack my runs in each week.  This race is to thank all the ladies in Run Mummy Run & Susan from Virtual Runner – without them I would have struggled so much more.

I can do this.  It doesn’t matter how long it takes me to cross the line.  It doesn’t matter if I have to walk some of it.  It doesn’t matter if I can’t keep up with the pacer.  I will cross that finish line and I will earn the same medal as the rest!


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