Running

Then I discovered that varying the intensity of my runs, and doing runs of varying intensity (intervals / fartlek / pyramids etc...) and I've take 10 minutes off my 10k and almost as many minutes off my 5k in 3 months.

+1zillion.

Go to a track and so some pyramids. They are horrible, but will make you faster.

Another one that helped me was 10 mins 'race pace', 4x400 Hard (20 sec rests between laps) - 2 minute rest - 4x400 Hard (20 sec rests) - 10 minutes 'race pace'

The second 10 minutes will be weird because you will be flying.

It really helps make you faster, pushes your LT up.