Researchers are still working to define the safe limits for vigorous exercise. The bottom line: if you work out to promote your long-term health and well-being, doing vigorous exercise for longer than an hour isn't necessary, and is actually counterproductive, says O'Keefe. Use these tips to maximize the benefits of moderate exercise:
If you like to work out every day: Don't do hard endurance exercise for more than one hour per day, and listen to your body: if your muscles are sore, consider building in a day of "rest" and swap hard-core cardio for walking or stretching. (It's OK to be active every day. Learn the Secret to Exercising Daily Without Overtraining.)
If you want to work out longer than 60 minutes a day: After the first 45 to 60 minutes of vigorous exercise, switch it up by doing yoga, strength training, or lighter activity like swimming--and don't race.
If you're already training hard: Researchers don't know for sure whether cutting back on sustained endurance exercise (i.e., running more than 25 miles a week for the past 10 years) can undo the damage done, and improve a person's health. (O'Keefe's guess is yes, though, based on related animal studies with promising results.) If you typically wake up with low energy, see no improvement in your fitness, have you lost your appetite, or have begun to think of workouts as a chore, you might have reached your personal threshold. Use common sense and cut back; like your muscles, your heart may need a day off from daily vigorous exercise. You don't need to lay around, but stick to walking or yoga instead of your regular workout for one extra day each week.
If you want a workout that helps you live longer: Sprint for 20 to 40 seconds, then let your heartbeat return to normal, and repeat five to eight times. According to O'Keefe, high-intensity interval training can improve your fitness without taking a long-term toll on your health.
If "run a marathon" is on your bucket list, no matter what: "People do a lot of things for reasons besides living longer, like jumping out of airplanes and racing cars. We're not saying those are bad, but they're not for your health," says O'Keefe. The same goes for marathon running. There's no firm information that running a few marathons is going to hurt you. Just know that competing regularly (i.e., running one race per year for a decade) won't promote longevity. (Running a half-marathon is a good step toward training for 26.2 miles--and a big accomplishment on its own. Try our 10-Week Half-Marathon Training Plan.)race.