I make it through an aerobics class, but can't run more than three minutes.

The wall you're hitting is likely not in your head, but in your legs. "The difficulty you are facing involves the principle of 'specificity of training,'" says Daniel Vigil, M.D., a team physician for the U.S. track and field team. "For example, even though good bicyclists are extremely fit, they're not necessarily good runners." Why? Because in cycling, your leg muscles—mainly the quadriceps and calves—are being recruited in a pattern specific to cycling; in running, the primary leg muscles used are the hamstrings. More importantly, "When you train extensively for a specific activity, it enables your body to increase blood flow, oxygen delivery, and nutrition transport to the muscles used during that particular activity," explains Dr. Vigil. "So you may be very conditioned when it comes to aerobics, but your muscles are not equipped to run with the same level of stamina, because they're not trained to do so." Aerobics classes, unlike running, consist of intermittent activity, and, says Bob Glover, a running coach for the New York Road Runners Club, "when you run for fitness, it is best to run at a steady pace." If you want to increase your running endurance, Dr. Vigil's prescription is: "More running and less aerobics." To ease into running, Glover suggests starting with 10 minutes of walking, gradually picking up the pace as you go until you are running at a pace where you can converse. "When you feel like you're starting to struggle with the run, interject a brisk walk break and then start running again," he says. Do this for 20 to 30 minutes. "After you get the hang of this routine," Glover assures, "you'll gradually be able to run for thirty minutes nonstop."