To prevent an ankle fracture, make sure you take proper precaution on uneven surfaces or when walking across areas with a risk of falling. If taking park in sport, make sure you complete a warmup exercise beforehand; these will stabilise the muscles and mitigate risk of injury.
If you choose to run on hills, make sure the surface is not too slippery, and avoid running too fast downhill. Take time to ensure all your clothing/wearables are suited for your body, especially your shoes. You can also improve the durability of your bones by incorporating calcium rich foods, such as cheese and green leafy vegetables, into your diet.