Coach Teresa has sound advice for improving your swimming.
Beginning with a self-taught course, and then finding a Coach in your area or one you can meet a few times and then keep in touch digitally is a great way to learn the proper technique for you. Many of the people that I coach come to me through that path. That way you are aware of the basic TI principles, and can improve with on-hands, individual technique coaching.
I completed my first 1/2 IM at 51...although I did have a base of training and shorter races, it was time!
Good luck with your training. Let us know how it is going!