If she's truly a dear friend and it's a relationship you value, why not say, next time it comes up, "Hey, we've talked about this several times, and clearly, we just aren't going to be able to see this eye to eye. I feel like you keep wanting me to defend this to you, and that's kind of annoying and upsetting. It seems like my decision upsets you for some reason, but I can assure you I'm not doing anything risky or unhealthy, so that concern isn't about me, really. And that's okay, too. It's okay that you don't think it's a choice you'd want, but I've come to this point carefully and thoughtfully, and as friend, I'd hope you could accept that we see this differently, and just be excited for me, just as I'm excited for you when you make choices that make you happy but that differ from what I'd have chosen for myself. Is that something you think you can do? Because I really don't want to keep having this same conversation, or to have this create an issue in our friendship."
If she can't do that, when you spell it out clearly, then she's a shitty friend. She doesn't need to understand, but she needs to respect and support.
Playing games and being semi-dishonest by saying you aren't retired when in fact you very much consider yourself to be retired is silly, and I think it lessens you, especially when you do it with people who are supposedly important to you.