I have started throwing in a few long hills into my training, they are agony and slow you down to a crawl but as soon as you go over the crest of the hill the pain goes and the legs feel great there on. This loop is part of my hard hill session and will form the bulk of the mid-week runs to try and concour hills and keep improving my flat 5k time.
Next week I start my first hill interval session. Essentially you run a 10-15min warm up run and then run up a hard hill of over 30metres long at a fast pace over and over for 5 times. This is absolutely excruciating but it is beneficial to boost your leg muscle power.