This workout is a mix of full out sprints, with then a jog/walk.  Go as hard and fast as you can for 20 seconds, then follow with 40 seconds of a jog/walk.  1 interval = 20 seconds of sprint with a 40 second jog/walk.​

This workout can be done anywhere you have room to run (either outside or on a treadmill).  

Then again, even when you don't have room to run you can do this workout by jogging and sprinting in place! Remember, where there's a will...there's a way.