Just start doing it, and do it a couple times a week. Push yourself as much as you can, if that is 1.5 km, then start with that and at some point you will be able to run a marathon (that probably takes a couple of years).
Always make some goals. Like, 1 more km each week or something.
As said above you need to train your lungs, as well you need to practice proper breathing. If you want to boost the distance you can run, dot go for endurance to train. Short sprints will build you faster than long endurance. Pick a short distance, and go all it in your sprint to the end and back. Every three or four days add one more half cycle (first day you all out sprint to te distance, few days later you go that distance and back, few days later you got that distance and back and then back out, etc). This will build your lungs and legs much quicker.