Have ridden a bike for years but also did other things for a good workout like run, swim, etc. Need to save the joints now so have really turned again to biking. Does anyone know how many miles or how long one needs to ride to get a really good cardio workout? I live in an area with fairly steep rolling hills so when using my polar heart pacer found it really accomplished an interval-like workout. I usually ride about an hour pretty fast without killing myself going up hills. Thanks