If you're more accustomed to running indoors on a treadmill but have decided to take advantage of the weather by running outdoors then you may want to take it slow at first. Running on a treadmill inside and running on the sidewalk outside are very different. One is not necessarily better than the other, but each requires different things from your body. Simple differences like wind and uneven running surfaces can make outdoor running a bit tougher, especially if you factor in heat.
The best way to make the transition from indoor running to outdoor running is to take it slow — don't expect to run five miles with ease outside if you're used to doing so on the treadmill. You may be ready aerobically, but it's good to give yourself a few weeks to get your body used to the added impact on your joints.