Certain types of training sessions can improve running speed:

 - Interval running
 - Fartlek running
 - Hill running

Basically going for high quality / high intensity sessions will do much more to improve your speed than lots of junk miles, i.e. long, slow runs (assuming that you want to target speed over a relatively short distance, e.g. 100m - 1/2 marathon).

Examples of running interval session plans (from my running club's website at http://www.serpentine.org.uk/pages/tuesday_threshold_sessions.html):

- 2 sets of 10 ( 1min @ 10k pace; 1 min @ tempo pace; continuous running); 2 mins recovery
- 2x 5 min (50 secs); 2 x 4 mins (40 secs); 2 x 3 mins (30 secs); 2 x 2 mins (20 secs) all at LT pace
- 12 x 600m @ LT pace; 40 secs recovery 
- 6 x 1 mile @ LT pace pace; 60 secs recovery
- 2 sets of 10 ( 1min @ 10k pace; 1 min @ tempo pace; continuous running); 2 mins recovery
- 2 x 5 min (50 secs); 2 x 4 mins (40 secs); 2 x 3 mins (30 secs); 2 x 2 mins (20 secs) all at LT pace; 4 mins rest; then 1 mile as fast as possible
- 12 x 600m @ LT pace; 30 secs recovery 
- 6 x 1 mile @ 10k pace pace; 90 secs recovery

Note that these sessions are designed to improve speed at races between 10K and marathon distance.