By careful selection of arrows, a 35# bow can be as effective for shooting targets as a 40# bow. For hunting, it would be nice to move on up to a 40# bow at some point.
Do you have any pain? If so, does shooting the bow aggravate it? If you don't have any pain, and it's just a matter of being weaker and less flexible than you used to be, then shooting the bow, combined with stretches that they probably gave you as part of your rehab program, could help you to recover your strength. It is important that you shoot the bow correctly, using your back muscles. This will enable you to shoot a heavier bow, and also help keep your shoulders from being injured.