The fitness center for the BWI is over in the Villas area, a one minute walk from the lobby. It is a nice center with plenty of machines. Most if not all are CYBEX. They have treadmills, stairmasters, bikes, and the usual assortment of other machines for abs, arms, etc. They DON'T have an elliptical machine
There is a fee to use it, unless you are a DVC member staying on points. You can by a length of stay pass. I'm not sure what the fees are, sorry.
There's locker rooms for men and women, plus private treatment rooms that you can schedule a massage in.