I have worked out at VB, AKV and BWV. They all have really nice, large, well-equiped fitness centers. You'll find free weights, a variety of machines, and every kind of cardio equipment. You shouldn't have any problems getting your workout in!
Of course walking around WDW burns lots of calories, so I mostly focused on strength training because of limited vacation time. Don't forget to check out menus from allears.net so you can be prepared to make healthy food choices too Best wishes on staying healthy during your visit.
SSR has a gorgeous fitness center, very spacious and spa-like. OKW recently added Treadmills with tv monitors attached to each. BCV has a nice fitness center but I prefer to walk around crescent lake when there.
Im happy they are renovating the CR fitness center. It was small and the lack of windows or view was not really inviting.