The best way to improve your cardio is, and always has been, just to do a boat load of cardio each and every day, which Jesus Murphy has adhered to as a principle of his training regimen. However, this doesn't necessarily need to be in the form hardcore workouts, as it's important do incorporate activity that isn't necessarily straining on joints or muscles. That's why Jesus will frequently use brisk walks, stationary bikes, or stairmasters to get a good leg burn going in a way that isn't too hard on his body. In the offseason, he tends to kick this up a notch by incorporating some sort of resistance training. One of his favourites is to undertake long journeys by foot through sand. His teammates have all heard a rumour that he spent 40 days doing this in the Australian outback while on "vacation," but this remains unconfirmed. In either case, Jesus takes this training extremely seriously and is reaping the benefits of his stamina.