Thank you all for your responses! I was surprised (and very happy!) to see so many responses so soon! I appreciate all your advice and will definitely keep it in mind. :)
I'm going to look into yoga and pilates for sure and also start getting into some running to get my cardio going. We're getting a dog in a few days and the little guy could probably use some cardio as well. XD My husband is totally on board with starting an exercise regiment, so I have even more of his support as well.
I don't feel as old as I did when I first wrote! I'm glad to see that riding is in fact something that anyone can be good at at any age. As I said, I'm used to seeing kids starting young and the adults I know being life-long riders, as well as the general belief that to be good at any sport, you have to start from childhood. It's a huge relief to see that there's many of you who are perfectly successful and started at my age or older.