As I said in my original post, speed is my ultimate goal. Fitness, loosing weight and generally being more healthy are all great by-products of running a lot, however my ultimate aim is to get faster (10k in under 40 minutes is my dream).
One thing I have been doing a lot recently is interval training. This is a really great way to increase speed in longer distances. In the past I have found that this has increase my overall race time significantly and I would recommend at least 2 interval training sessions a week.
I use the built in Interval training function on my Garmin Forerunner 305. I sprint for 30 seconds, rest for 30 seconds - repeating this 10 times. The watch is great for this excercise but not essential. All you need to do is pick a point in the distance roughly 30 seconds and sprint to it giving it everything. Then stop and run back. Another method people use is to walk back to the original start point, then start again but I feel this let's my heart rate drop too much.
I would definitely recommend intervals, if you are trying to loose weight or get faster at running. I am going to be doing a lot of this type of training in my quest to get under 40mins on my 10k.