Depending upon how active or sedentary she's been since retirement from racing, she may have lost a lot of muscle, so can look too thin when she's thinner. She's probably be just fine at 68 to 70#, especially if you can see the last 2 or 3 ribs. Although I don't consider 8 yrs. old to be particularly OLD, in my experience, seniors can lose weight quickly... sometimes frightfully so. You might want to try her out on supplements like glucosamine that help with joint stiffness.