Welcome to the community newlupie. We are 1 big family. We come from all over the world so are on different time zones. I have fibromyalgia from Ohio USA. I walk. We are going into Spring so it is getting warmer. I don't walk so much when it is cold. I am thinking about joining a gym so I can get more exercise. I admire you for running in a half marathon.
I'm a Lupie here too. I also have osteoarthritis in my hips and back. I'm 46 newly diagnosed in the last year but have had it for many years. I try to work out three times a week doing squats, a little ballet, a little yoga and when I can handle it some hip hop. Lots of stretching and walking. If it's affecting your joints, high impact is not good. They say swimming and biking is really good. Neither of which I enjoy. I used to be a rock climber, hike in the mountains, work out hard daily. I miss it but I do what I can. Lupus has affected all my joints pretty bad.
Best of luck to you. A half marathon is very ambitious. If you have the endurance now to go through that. I'd ask my Dr for or look for program with trained coaches to help you keep your strength a d not to hurt yourself. I am in physical therapy. Doing aqua therapy. It's a lot better on the joints.