What do you do if your g/f is a lazy beatch?

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My g/f could do with a bit more exercise. Really. So I've got her to exercise when I do a session at home. I have an exercise bike, rowing machine, and some weights. Now - I put loud music on and really get into it, but the moment she starts getting tired she stops. Usually she does the exercise bike, at something slightly above light joggin pace. In fact, she hardly gets a sweat going. I keep saying that you are SUPPOSED to get tired, that's the point godammit - but she doesn't get it.

How do I get her to understand how to exercise properly?