You might try adjusting your pace some. If you are 100% spent after your runs you should slow down a lot and see if that helps. Alternatively, if you aren't dyi

You might try adjusting your pace some. If you are 100% spent after your runs you should slow down a lot and see if that helps. Alternatively, if you aren't dying at the end of your runs maybe try and run a little faster. A fun workout to do is to start slow and warm up for 1-2km then pick out a landmark that is relatively close like a telephone pole or a tree and then sprint to it. Once you get there show down to a recovery pace and pick out another landmark at which you begin sprinting again. This is called a "fartlek" it is characterized by sprints and recovery periods that are irregular distances. You are not sprinting for 400m every time.