I have found this tricky to implement because of my particular health difficulties but the evidence is good across populations that doing at least some of your exercise as an 'interval' format has increased benefits. And I have found this to be true for myself. It seems to be especially useful for shifting you off a weight loss plateau. Of course that's how NHS C25K works for running in the beginning. After that I have used the NHS C25K+ podcasts Audiofuel's Pyramid 180 and Runkeeper's Run for Fat Loss programme. I have to be really careful and not push my hardest or I undo the effects by being laid up worse than usual, but it still works. The Pyramid 180 is particularly good as it is short so if you don't have much time or feel not in the mood, it packs a lot in.