I know we all have different metabolism, in my case an hour to an hour and a half of bike riding burns more calories than jogging 2 miles in 20 minutes even though jogging feels more intense. My point is and I feel that the cardio effects start to take place after 45 minutes to an hour or more of continuous exercise.
I could never jog longer than a half hour without feeling physically and mentally uncomfortable where in a bike on a good day I can ride up to 2-3 hours.
Then again, any exercise is better than none.