Well running outdoors is a lot different from running on a treadmill, it is generally much harder and requires more effort. When you are outdoors the surface isn't level and can be bumpy at points, also you will be running on a different surface which may be very soft and hard to run on like grass or tough and hard to run on like concrete. So the best thing would be to wear a good pair of running shoes or training shoes and wear shorts to make it easier to run as they don't weigh as much and won't absorb much water like longer pants/trousers and make it even heavier. Also, don't just start running long distances straight away, just start with what your comfortable with and build up from there and you will finally get the hang of running long distances no problem.