Every spring if I have not been faithful to my exercise program, I find out very fast when I start riding! Ouch!
As far as cardio goes, try posting for 30 minutes and than take your pulse rate. Like anything, the more fit you get, the more resistance you need to continue to challenge yourself. While riding shouldn't be the primary exercise in a fitness program, it is exercise. Well, that is if you are an active rider and not a passenger.