Subacute rehab means that you will be taking care of patients who are fairly fresh post-ops and aren't ready to go home until they have some inpatient therapy. When I worked SAR, I had lots of IV antibiotics, tube feedings, dressing changes, and med passes. It's hard work, and you will be very busy, but you will gain experience, and that's the main thing. You will need to be able to depend on your CNA, so don't be afraid to ask him/her to pull their share of the load, because there will be a lot of assisting patients to the bathroom, getting them up for meals, etc. and if you are doing the aide's work you won't have time to do your own. You need to have good assessment skills as well, because you will be responsible for watching for post-op complications such as infections at a surgical site and notifying the doctor if necessary. Having a good team helping you is key in this type of nursing, because with 20 patients, your ears and eyes can't be everywhere at once.
Good luck and congratulations on getting your first RN job. I hope it goes very well for you.