Here's great cardio workout that will help you build your endurance. This routine contains two exercises that will help you gradually increase your stamina.
1. Stationary Bike
Time: 10 minutes
1.) Warm up for riding at 50 rpms at level 1 for two minutes
2.) Ride at 65 rpms at level 3 for the remainder of the 10 minutes.
3.) If you can't finish the entire 8 minutes, slow down to 50 rpms for one minute to recover.
4.) Cool down for 1 minute ride at 50 rpms
1.) Warm up for 2 minute walking briskly.
2.) After 2 minutes, jog at 5.5mph until you can't anymore.
3.) Walk briskly for 2 minute to recover.
4.) Repeat this sequence until you finish 20 minutes
Note: Eventually you should be able to jog the entire 20 minutes without stopping.