You could try making your walk "power walk," because

yoga is mostly muscle conditioning and stretching and doesn't have a cardio component (unless it's bikram yoga). Power walk is one of the cardio exercises with the lowest risks. You could google to see what are the key points for power walking. You could also try going for a power walk first, which warms up your body fully, and then take yoga. I generally do cardio first, then strength training, and end with stretching. I think there is research supporting this sequence of working out.