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According to doctors sex is a very good cardiovascular work out. Unfortunately for some of us it only lasts as long as we can last on the Elliptical which is about 5 minutes (I am being generous).
I would think though you would have to do more work than laying on your back, but maybe Draya has had enough experience to know better than I.
If you want a full view of what she would look like on here back you may view these NSFW photos.