Summarize this article in one sentence.
When you’re first getting acclimated to working out outdoors, it’s best to start with roughly 15 minutes of moderate exercise at a time. As your conditioning improves, you can begin to add 2-3 minutes to each session. Take plenty of rest breaks and be careful not to try to do too much too soon.  Pay close attention to how you feel after each period of activity. If you detect that your performance is dropping off, play it safe and decrease your intensity or take longer breaks. It typically takes about two weeks for the average person to become heating acclimatized. Fill up on cold water before you set out to bike, hike or jog and plan for frequent hydration stops along the way. Keeping your tissues flushed with water is crucial if you’re going to be slogging it out in stifling conditions—elevated temperatures will cause you to sweat constantly, even when you’re not actively exerting yourself.  Dehydration can be deceptive. Replenish your fluid levels at regular intervals, whether you feel thirsty or not. Always carry a water bottle with you, or make sure there’s another available source of water nearby. Sports drinks replenish not only fluids, but also essential electrolytes which are needed to power your muscles through a workout. After your first couple of weeks in a new environment, crank up the duration of your activity to around an hour at a time. This will soon become easier, and you can begin spending even longer periods outdoors. If your goal is to acclimatize as quickly as possible, plan on building up to least two hours a day.  Once you’re able to comfortably stay out for two or more hours a day, you’ll begin to feel much more at ease in motion and at rest. Raise your tolerance by finding an area with some light shade or stripping off non-essential items of clothing rather than seeking refuge indoors. Monitor your heart rate and breathing patterns vigilantly and be ready to call it a day if they start getting out of control. Even if you’re an elite athlete, there will come a point when your body can no longer keep up with the demands of working out in the heat. At this point, your continued efforts can go from difficult to dangerous.  Listen to your body, not your exercise journal. Stop what you're doing and seek out a shady place to rest when you feel the heat getting to you, even if you haven't finished your session. Consider splitting your workouts up into multiple short sessions to reduce your risk of overheating.
Perform small bursts of activity. Drink plenty of water. Increase the amount of time you spend outside gradually. Don’t exceed your limits.