Well, horse therapy aside...
I have met a handful of people who have had hip and or knee replacements who are quite active. Probably easily to the rigors of low to medium volume EMS.
That being said, they were people who made their careers in extremely physical professions and were in quite good shape to start with. I think it will depend on your overall health and fitness, a little bit of luck with your body's innate healing ability, and probably most important, a skilled surgeon who believes in rehab to a quality level of activity and not just restoring minimal function and putting you out to pasture. Uusually those surgeons are heavily involved in sports medicine and usually at an academic medical center. (Which with rare exception is better medicine than a community hospital anyway)
My observation is that rehab to that level (if possible from your health/fitness standpoint) is quite painful and requires considerable effort. It will certainly not be easy. There are also many non traditional roles for paramedics now. Hospitals, amusement parks, clinics, etc.
Your doctor should be able to give you an honest appraisal both before and after surgery as well as how your physical therapy progresses. BUt like with all things, if you need a major surgery and have all kinds of comorbidities, your chances will decrease. Depending on your current health status, it may be suggested by your doctor to improve your health pror to surgery. You may want to ask about it if he/she does not bring it up.