There is a 'pyramid' programme that I sometimes use in my fartlek training. This is when you run a few short sprints with recovery in between. basically 1 min jog, 1 min run, 1 min jog, 2 min run, 1 min jog, 3 min run... all the way up to five minutes then back down again 1, 2, 3, 4, 5, 4, 3, 2...