While speed workouts can make you faster, they can also slow you down.  Too many runners think that going to the track and running intervals will make you faster.  This is partially true, but it can also lead to fatigue, injury and eventually cause you to become slower.  Just this week I encountered a runner in the office who was more than likely pushing his pace too much which is preventing a hamstring injury from getting better.  Doing all the PT in the world will not fix the injury the source of the problem is running too hard without recovering.

Read this great review of how base training gets you prepared for speed workouts which should follow it.

[SPEED WORK DEFINED: For this article, speed work refers to reps performed at your VO2 max pace (pace for an 8- to 10-minute race) or faster. A typical speed workout in this range would be 6 × 800m repeats at VO2 max pace with a 400m recovery jog between.] He got fried. He knew better, but like many athletes who are running fast, he lost the forest for the trees.

Source: When Speed Work Sabotages Your Gains