I second the walk/trot with horse and/or dog suggestion - especially if you have a Bossy Collie, who refuses to let you slack off
Another thing that's good for cardio is biking, especially if you can use it for trips (like home-barn) that you might ordinarily drive, so you're replacing drive time with exercise time. (And saving gas money
Either way, just remember that you have to push yourself a bit to get the most benefit. (Look up target heart rates.) For biking, getting a speedometer/computer works for me. You find yourself trying to beat your previous times...