High Intensity Interval Training
The Tabata interval training is simple and short, but produces remarkable results. It has been shown to improve both anaerobic and aerobic fitness in trained athletes. It is one of the most efficient work outs in terms of improvement for time committed, as well as being a lot of fun. Paradoxically, this type of high intensity interval training also produces endurance improvements[1].
The Tabata workout is 8x (20 seconds maximum effort with 10 seconds recovery) for a four minute workout[2]. Before starting the Tabata it is important to warm up thoroughly as this work outs a lot of stress on the body. The 'maximum effort' nature of the Tabata intervals creates significant risk for injury. I would strongly suggest that you start doing the Tabata structure, but instead of doing the 20 seconds ‘all out’, do them as comfortably hard. Over the course of a few weeks, build up to doing them as fast as you can.
Bonus tip - This workout can not reasonably be done on a treadmill. This is partly because the intervals are very short, with a dramatic change in pace between interval and recovery, so a treadmill acceleration becomes a problem. The other issue is that 'all out' is not a pace you can program into a treadmill. (Thanks to Emily K for this)