Consistent training is the key.
Follow my advice and be consistent. Increase your stamina while avoiding injury. Gradually ( don't over do it) build up your personal aerobic ability through running and other extra aerobic exercise. As your muscles become stronger, aerobics bring more oxygen for stamina. Add 3-4 runs each week. One of the runs will be your usual comfortable run and the other three, make these runs around 35-45 minutes and just enjoy them. Don't try to speed, your fine tuning and gradually building up stamina.
For your usual comfy run, add a mile onto the distance which you usually run. Don't race, just maintain your usual pace. You can think about speed later, after you have built up your stamina.
After about two weeks, your three shorter runs will become too easy. You will feel it and you can move these runs to a "tempo level". Choose a harder but comfortable pace, run for between 35-60 minutes. Lactic acid builds up in your blood and brings on fatigue. You're training your body to clear out this acidic build up and stave off fatigue.
This regime will challenge your body, as you gradually increase your distances and pace, have a good stretch routine. Within 30 mins of finishing a run, eat some quality food or try protein bars. Feed your blood with nutrients while your muscles develop. Work on your running style. Efficient running will also take you further.
Psychologies important. You can do this, go further and faster than you did before. Keep reminding yourself you will succeed.