This image shows a comparison of my mile splits from my two quickest 15 mile races. They are both run over the same course and the notes from my training diary for each are as follows:

Very undulating, ideal conditions, exposed. 3 large hills, twice.
Very undulating, very strong headwind up and down first hill, exposed. 3 large hills, twice.

The image will allow me to pick apart each mile, analysing where I could improve or what adjustments I need to make to my training. However, it also suggests that it is probably just as well that I don’t own a Garmin*

*other GPS devices exist