It is strange how things turn out - last week I did a 16K long run at 8:30 mins per K and struggled with it - today I did 17K at 8:00 mins per K ( 30 seconds per K faster!) and felt that it was just a bit "easier" . I did wear different shoes - but apart from that, everything else was much the same. It became obvious quite early on in the run that my pace was faster than last week - and couldn't really slow it down. I have no idea why it was 30 seconds per K faster today -- there is a physical place at which the GPS reading for the two different runs were about 300 metres different to one another - a 300 metre error over 15K isn't outside the realms of reasonableness!! I guess it is why we can really only use GPS watches/Apps as a "guide" .