In cardio training around the world, HIIT or high-intensity interval training, has been a big hit among serious fitness enthusiasts.
This form of cardio mixes intervals of high-intensity exercise such as sprinting with intervals of either low-intensity exercise such as walking at a slow pace or complete rest.
HIIT was developed decades ago by track coaches to better prepare runners. Studies showed the short bursts of high intensity training gave better all around results in cardio and strength.
With HIIT, you'll be running or cycling or whatever at high intensities for brief stretches, but the net effect when all's said and done is better results in less time.