I'm often cramped for time and I want to get in a good cardio workout without doing my usual 40 min. walk.

Is a 40-45 minute ride with a speed of 18 mph enough of a cardio workout or do I need to go longer at a slower speed or shorter at a faster speed to get a good workout?

Is 40-45 minutes of biking enough of a workout at all if I'm wanting to get in a good cardio workout?

I eventually want to build up to long rides but right now my max is an hour 1/2.