I have a 14-year-old mixed breed who is still incredibly active. She'll run for miles quite happily - if I walk four she'll run at least twice that. I've always been pleased she's so fit, but was just wondering as she's getting older should I be limiting her at all for the sake of her joints? She sometimes seems a little stiff of an evening, but is always rearing to go again by the next morning. We give her joint supplements. My previous dog developed severe arthritis at this age, and I really want to avoid this.