I Started doing speed/pace sessions and saw speed improvements gradually. I am using the speed/pace sessions now to see if I can get my 10 and 8km times down. For example:
4 x 1km at goal pace with a standing 1 minute recovery
10 x 30 second with 30 second jog recovery
2 x 2km at goal pace with a 1 minute standing recovery
Also 3km distance time trial. In the Time Trial I just see how fast I can go for 3km. Yet another one I use is a 15 minute 'effort' session.
It works quite well. Do these pace sessions every two weeks and mix them up. ie have one week slow easy, and the next include a short speed and a longer pace session with a slow run in between. I ran for ages with no speed sessions at all (nearly a year) when I first started, and as soon as I started these pace sessions, I suddenly was cruising at under 5min/km in my long easy runs..
Also don't be tempted to go fast all at once in a longer run. Build speed in pace sessions, not long easy runs. The long easy runs are for building endurance, the short speed runs to build pace. Also as far as distance/endurance goes, be careful, and build distance each week, maximum of 10% increase. These tips are supposed to help you avoid injury.
Every now and then go for a longer distance time trial or a race, but begin running a little less in training two-three weeks out as a 'taper'. In the race you will be fresher and will handle the extra effort you make in the race a whole lot better.
I have to keep reminding myself that every time I run it is _not_ a race!! Push myself, sure, seek out hills, but in long runs don't go for a personal best each time :) It is all about building fitness and strength, and tapering for a race to see how fast you can go. The training is supposed to be good for you not wearing you out.