Hills - your choice
Hill Repeats: Find a hill and do multiple repeats on the up side of the hill, turn around run down smoothly-don't pound and repeat when you get to the bottom. Try to get a little quicker each time up. Focus on good arms and, a bit of knee lift and pushing off all the way through your toes.
Hilly Route: Just find a route with some hills and when you come to a hill attack the upside of the hill and the practice running the downhill side smoothly and not jarring and braking on the way down.