You need to raise your heart rate to a certain level called your Target Heart Rate and keep it there for 20 minutes to increase your cardiovascular fitness.
To figure out your Target Heart Rate, use this formula: (220 - your age) x 70%.
For example, if you are 28, it would look like this: 220 - 28 = 192 x 70% = 134 beats/minute.
To check your heart rate during exercise, count your pulse for 10 seconds, then multiply by 6. Compare this to your target rate.
A quick and easy way to assess your level of exertion is to do the "talk test" while exercising. Rate your ability to talk on a scale from 1-10, with 1 being no breathlessness and easy conversation and 10 being extreme breathlessness and inability to talk during exercise.