Once you have a good base of being able to run for about an hour, you may want to start adding in strides. Strides are short bursts of swift running for 80-150 meters.
First you always warm up with a mile at easy pace, a pace you can still talk (conversation pace) and run at the same time. Then do some strides: speed up for the next 40-100 meters, (max. heart rate) then slow down back to your conversation pace. Recover with slow jog or walk until your heart rate comes back down (below 120) then repeat stride for 6-10 times with recovery in between.
Purpose of Strides:
Promote good running form
Exercises your fast twitch muscle fibers