I've run cross country at my high school, and while I've always been in good shape from basketball and soccer, I found it difficult at first to run a steady three miles.
Most importantly, its basically up to how you push yourself, you have to WANT to run the distance. Always have an iPod or something to take your mind off your fatigue. If not, then count your steps or hum a beat or something in your head. Don't concentrate on being tired.
Make sure you breathe correctly. I've been told to breathe in through my nose, and out through my mouth. You usually get cleaner oxygen through your nose because it filters more pollution. Also, pace your breaths--take deep slow breaths for long distances, not short sharp ones. If you start getting tired, a few DEEEEEP breaths usually calm you back down. (Also, don't run with your head down. Keep your head up and focus on an object you want to reach). Never stop and walk, if you stop your jog then it's even harder to start back up again. And don't waste your energy by swinging your arms really far, but take long leg strides
Don't eat before or drink cold cold water. Try and set goals. This week, one mile. Next week, mile and a half, etc. Eat bananas or potassium after, and try and walk about 1/4 a mile.
What works for me: Singing a nice fast-paced hip-hop/dance song in my head to "keep my energy up" (or get pissed off about something haha), breathe deeply and at an even pace, focus on an object to reach, and tell myself to never stop running. In 2-3 months I got my 5K time down to about 19 minutes
But ultimately, you have to find what works for you. Experiment with different things and find your comfort zone