Here is a mini cardio triathlon workout in which each round increases by a certain distance to make it tougher as you go along.
It's a short but punishing workout if you push for a better time.
Here is how to perform the workout:
Use round 1 as the warm up if you are stuck for time otherwise the warm up can be 250m Row + 10 Cals Assault Bike + 1km bike.
I would record how long it takes to do the workout and then try to beat your overall time every workout until you cannot beat the time any more.