A round of golf might take more than four hours, but only a fraction of that time is spent hitting the ball. If you're interested in improving your stamina when you play, the cardiovascular portion of your fitness routine should mimic a similar stop-and-go action.\n"Successful golf requires short periods of intense focus and the ability to repeatedly execute fine motor skills during that time," says trainer Mark Verstegen, founder of the Athletes' Performance Institute and fitness centers. Improving your ability to activate and deactivate your concentration will better regulate your levels of energy, from the opening tee shot to the last putt.\nHOW TO INTERVAL TRAIN\nSteady cardio training -- like running at a consistent pace on a treadmill -- could be counterproductive if you're trying to enhance your focus. Instead, take your current cardio activities and apply interval training to them (bursts of high-intensity action followed by performing the same activity at a slower rate). This should increase your cardiovascular power and improve your brain's ability to focus and relax on command.\n"You want to be able to work intensely, then drop all the way down in an instant," says Verstegen. In the pool, for example, swim aggressively for a minute, then tread water or swim slower for a minute. This works with any activity -- even strength training. You can build the muscles in your legs -- the key to a more powerful golf swing -- while making your heart pump, Verstegen says. To do this, line up a series of three exercises (see below), and complete one set of each without a break. Drink some water, rest a minute and repeat the series. This can work with your current fitness routine or with the routine Verstegen recommends here.