When it comes to wanting to get faster, there's only one speed: AS FAST AS YOU CAN! Sprinting at or above 95% of your max speed with every rep you do is critical for speed development. Reason being, is your body's central nervous system only adapts for speed when it is stressed at a high enough level. Basically, it can only recruit your large motor units and fast twitch muscle fibers when you are running as fast as you can. Anything less than 95%, and your body is essentially is getting more conditioned, but not faster.
So perform low volume, high intensity training with adequate rest (2-5 minutes) to maximize the effect of your training. Make sure to track your speed any way you can, either with a timing system or stop watch. If you are unable to do so, just make sure you're trying as hard as you can and make sure to stop training once you begin to feel fatigued. Keep slower, conditioning workouts separate from your speed days and you will see results!
Here's one study explaining the importance of high intensity (95-100% of max speed) when developing absolute speed.