Try
HIIT cardio (High-intensity interval training).
In short, you would run/bike/swim/whatever as hard as you can for 60 to 90 seconds and slow to a normal pace for twice that time.
For example, you walk/jog at 5 MPH for 120 seconds and you sprint like a mother at 9 MPH for 60 seconds.
Pretty hard stuff. Some people do something like 5 minute running/10 minute walk.
Experiement and see what works for you.