Right the way I did this was to like set yourself like a little 100M track and everyday Run as fast as you can over the 100M and eventually you will get a lot quicker over time this is how I trained to get into the army
Well my rugby coach and this site on running said that stretching well everyday can increase you're performance by up to 40%. Not being flexible and running is like driving wih your handbrake on.
Good luck, Greg.
Try running at diffrent speeds and distances evry day, for example, one day you run for 15 mins, so try changing your speed like, 5 mins jogging, 5 mins sprinting and 5 mins walking and then the same routine all over again.