Hills can frankly suck, but they can also be your friend for speed and overall strength. If you live near a hill, take advantage. If not, this would be a good reason to head out to an area such as McDonald State Forest or Bald Hill. Give this workout a shot, and mix it up as you get more comfortable with the format.
15 minutes easy6 x 2 minutes uphill
Walk or jog back down for 3-4 minutes to recover
15 minutes easy
You do this once a week, and over time I am confident you will notice a difference.