Here is a simple structure for cardiovascular exercise.
Effort Over Time
First, pick a single type of cardio exercise. Jogging, elliptical running, stair climbing, stepper, jumping rope, biking on an air-resistance bike (like the Schwinn AirDyne), etc. Avoid treadmills or other exercise equipment that demands you meet its pace unless you're willing to keep changing the pace as necessary. Avoid equipment that doesn't track anything and on which you cannot easily track distance or effort - such as spin bikes.
Second, set a specific total time to exercise - 10-20 minutes is generally good, but you can even drop as low as 5. This is your total time, including rest, from start to finish.
Third, pick an easy metric for you to track. These can be miles or calories on a bike or elliptical, calories or meters on a rower, skips with a jump rope, steps on a stair climber or stepper (or staircases, on actual stairs) - you get the idea. Easy to track, easily quantifiable.
Finally, each training session do the chosen exercise for the chosen time. Rest as needed, and set the pace at whatever feels good that day. You'll want to push yourself a little bit if you goal is fat loss, improved cardio endurance, or similar benefits - and hold yourself back if your goal is recovery cardio.
Each training session, track your metric. Try to increase it over time. Some training sessions you'll be flying - on those days, push yourself. Cook while the over is hot. Other days, you'll be run down or just can't get into the rhythm or pace - it's fine, just get some work in. Progress will be a wave, not a straight line, pointing towards success.
Readers familiar with Escalating Density Training will recognize similarities in this approach. Unlike EDT, however, it's intended for exercises where the resistance cannot easily be increased - jumping rope, say, or jogging. Or riding an air-resistance bike where resistance always increases and can't be modified directly, only increased by working harder. It's also a single exercise instead of alternating. And finally, it's not aimed at hypertrophy.
How does this work in the real world?
Quite well. I use this with air-resistance stationary bikes (using calories and miles over time), rowers (meters over time), and even running (distance over time). It makes for a good, fixed-duration training session (easier to schedule) and lets you rest as needed which adjusts to off-days, fatigue, especially good days, improvement, etc. It's an easy way to ensure clients get in some cardio, adjust it to their level that day, and see progress over time.