Simplest way, start increasing your running distance and speed a little each week.
Say, if you're running a mile now at 6.0, do that for one week. Then move up to 1.0 mile @ 6.5 or 7.0 for a week. Then either drop back down in speed .5 or 1.0 and increase to 2 or 3 miles for a week. Then increase the speed back up. And so on.
Or, keep your speed constant, whatever you're comfortable with now, and just add an extra mile each week to your run.
It's actually neat, once you get to a certain point when running somewhat long distances, you get to a point where you can probably run for a long time. For me, once I hit 1.5 or 2 miles, I can probably run for several miles. It's just that first mile and a half or so, that SUCK!! But then you get into a groove, and it becomes easy.
Now, take note, I'm not a runner, nor do I like running!!! I don't mind doing interval training (cardio) with fast running/sprints and walking or slow jogging. But I'm not expert when it comes to long distance running, compared to some others that probably know from personal experience. That's just my opinion!