Jumping into long distance running without building up to it is the surest way to repetitive motion injuries such as shin splints or stress fractures. Your lungs and your legs might not be at the same level of fitness for long distance right now. Start out walking the distances you want to be going. Once you’ve got that going, go the same overall distance, but run some, and walk some. Keep increasing (slowly) the distance you’re running, and soon you’ll be running the whole thing.
Just make sure to listen to your body. If it hurts, stop. If you’re just tired, push yourself. Injuries make it hard to run at all.
There are lots of running programs out there that can start you out on the right foot, so to speak.