I am in the same position as you - coming up on 50, healthy but the dreaded type 2 diabetes (tablet only) side effects has hit me and the old cock just doesn't work like it should anymore. For the past 3 years it has been going downhill. I tried viagra, gave me a splitting headache and only got him half hard. It's gotten to the point now where it is flat out getting hard enough to penetrate. In the end I bought a hollow strap-on and we use that and my wife seems to get off on that as much as she did when my cock used to be able to drill through walls :-) I'm by no means big, 5 inches back in the old days, and I bought a 7 inch strap-on to give her more of a thrill. Watching how wide her eye went the first time I slid it in to her was one of the biggest turn-ons I have ever experienced! She went crazy, grabbing my arse, and the leather strap, and slammed me in to her. She gets off on it so well, and once she comes it is my turn. If she lays on her back and puts her legs straight up in the air, keeping them nice and tight together, my cock is just hard enough to go in to her and stay there until I come. She still enjoys the feeling of me coming in her, and I think this tops it off for her. I like to think that she sees me as the "2nd man" in a threesome after the big cock plows her. There is no way I would ever share my wife, but it is a turn on wondering if she is fantasizing that she is being done by someone else and then I am going sloppy seconds.
So in closing, I'd say go for it! Buy the strap-on and give her the pleasure she deserves and then hopefully it will slao help you get hard enough to finish off yourself as well.