Your Choice of speed
Repeats: Pick a distance up to 400 meters and run it multiple times with the same distance for your recovery in between. Your recovery time should be to almost full rest. Each repeat you should try to get just a little bit faster.
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.
Surges: Pick a time or a distance to pick up your pace, and then your recovery will be twice of your surge. You could also use landmarks like telephone or light pole. Run one pole hard and then two poles easy, and then repeat.
As far as total distance goes you will probably know when you are done. As it becomes harder for you to speed up anymore, or it keeps taking you longer to recover, you will know you are done with the quick stuff for the day. Make sure you get in a good cool down to finish.
Have a great day. Sting