The Nike Sportband. Disturbingly honest. (Nike)

We all lie to ourselves.

Whether it's quietly thinking "I'll only have the one quick pint", or "I'll definitely organise my finances tomorrow", we tell ourselves things all the time we know might not be completely true.

This is very much the case with training. Some days a run just isn't as satisfying as others. The endorphins don't seem to be having the same effect, there's an oppressive drizzle soaking you through and motivation levels are at rock bottom.

At that point it's easy to rein yourself in, cut a run short, or not push yourself as hard as you should while the voice in your head tells you: "I've done a good run today and will do an even better one tomorrow." But you know you haven't.

However, with the deadline date of the London Marathon advancing like a Panzer division on the horizon, I realised I couldn't go easy on myself. If I cut corners my family and friends will see the result as I collapse pathetically at mile 10 and get trampled by runners dressed like Power

So this week I tried the Nike + Sportband. An impressive piece of kit, the band is paired with a chip which you place in the sole of your special Nike shoe (or perhaps into a hole gouged into another pair). As you run, the chip measures your pace, while the band records your time, speed, calories and distance.

Upon finishing what I thought was a good midweek run, I plugged the Sportband into my laptop and came to a realisation - this little chip is basically a physical representation of your conscience.

It's inescapable. Every time you slowed down, every metre you covered - it's all displayed on screen in a simple line graph. Mine resembled a cliched view of the stock exchange board: a few dips and spurts followed by a shocking decline that would see most traders consider exiting a skyscraper via a window.

The Sportband's honesty is brutal. You can't tell yourself you've done well when it's in black and white in front of you, like a school test paper with "See Me" scrawled angrily over it in red pen. It's oppressive at first, as if you are being followed on your run by the giant pointing finger from the National Lottery adverts, although instead of "it's you", the booming voice is repeating "failure".Or perhaps I'm exaggerating.

Of course, the highs follow the lows. Those good runs, where you pushed yourself that bit harder and really saw progress, are spelled out with Nike's pervasive "Just Do It" terminology helping to foster the odd moment of tentative hope that the marathon distance might just be attainable.

All things considered, thanks to the Sportband, even if everything goes wrong on 22 April race day, I will know the precise mile I collapsed at. Which is nice.

Nick is fundraising for Amnesty International and hopes to raise £1,800. To sponsor him visit his fundraising page.

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