When I had ACL surgery 20 years ago, it was my first time in PT. I did five or ten minutes on a bike, then an elliptical and then on several other machines---or on the table---clam shells, for instance. I was amazed that at the end of four weeks, I was much stronger than before.
So, strengthening muscles is not about an hour a day of exercise. 30 min is plenty of time . And if you are working--fit exercise into your day. Walk up the stairs instead of an elevator, do leg raises when you are standing around. Clamp in your stomach when you are standing in line for anything. Do clam shells and other leg exercises while watching tv. Just do a little every chance you get---it will add up.