If you've been running all Winter on the treadmill at your gym, you must be psyched to hit the pavement or trail outside. The warm weather may inspire you to lace up your sneaks and continue the same running routine, but remember that running outside is a lot different than running on a treadmill. The moving belt on a treadmill helps you out a bit by pulling your feet back for you, which means you're exerting less energy. I also find that the even pace the treadmill creates is easier to follow and because it's harder to keep my own pace steady when running outside, I end up running faster than I should and get tired before my workout is finished. Plus there are natural obstacles to deal with like heat, wind resistance, rain, rocks, puddles, tree branches, bugs, and hills.
Please don't think I'm discouraging you from running outdoors. Definitely be excited about it, but remember to ease into your runs in the beginning, and don't be so hard on yourself. If you're used to jogging 10-minute miles on a treadmill for 30 minutes, maybe you decrease your speed on your first few runs. Or you can maintain your speed, but run for only 20 minutes. Give your body time to adjust to running outdoors, and gradually work your way up to what you were doing on the treadmill.