I think you would benefit from a trip to see a physiotherapist! They can provide guidelines for your particular needs that can help you deal with your injuries, and not exacerbate them. Have you visited your doctor to be cleared for exercise? With old injuries and such, this is especially critical, although everyone needs to.
There is also a Limited Mobility lifestyle section that includes some seated cardio workouts: www.sparkpeople.com/resource/lifestyle_cen