The laws of physics limits the top speed of an equine type quadraped to about 45 mph. Marigold’s legs are a bit longer relative to her body, so she could probably run a bit faster with a longer stride length than any horse. Breathing is timed to the stride. If they stride too quickly they can no longer breath. Increasing stride length is than the only way to increase speed. To run faster than a car Marigold would have to run with an enormous bounding stride well beyond the ability of any horse.