Running long distances requires a "base", which is several months of running just to get in shape. Until that time has passed, don't worry too much about distance. After that, to increase distance, here are a few choices.
1. Run slower, but run longer. Decrease your pace by about 1/2 minute per mile, or even 1 minute per mile. The day before this, take it a bit easier. You'll need the energy.
2. Run some intervals, or as the Swiss would say, run fartlek-style. This is running but changing the pace quite often, and throwing some faster sprints in it. Intervals are easiest when done on a track. You can get better time on splits, rather than perceived effort. The clock won't lie.
3. When out running, every mile run 100 yards at race pace, which should be about 1 or 1.5 minutes a mile faster than your training pace(approx.).
I'm sure other people have different preferences, but this might get you started.