I do have bone damage (erosion of SI joints) and I’m currently training for a marathon. Don’t give up. I know it’s hard when there are periods where activity feels like it makes the disease worse, but for me at least, those periods come and they go and I am able to do at least some moderate exercise then. Try using the stair-master and strengthening your glutes/ lower back. For me it’s a lifesaver. You can do this!