To improve stamina, you have to increase your mileage.
Stamina is the capability of sustaining to run for long distances. In other words, having good endurance. A very good way to improve and to clock your mileage is doing long distance jogs. This is also called "easy runs".
Long distance joggings need not be over 70% effort. A 40-50% effort is sufficient. The main aim is the run long distances, not fast speed. Thus, in this way, your stamina will be improved.
You can start with 2KM easy runs, increasing to 5KM depending on your body's adaptation.
Alternatively, besides running (which you might not like), you can choose to swim. Swimming for several laps without stopping helps too. Cycling for long hours might be another way too.
Some mentioned Intervals. Yes, but they'll come useful but only after you have sufficient mileage. Interval only helps to improve your speed which means your timing; it doesn't help in your stamina building.
My advice to all: if you want to do well in 2.4KM, you ought to have trained a distance of 2X the distance, which means 4.8KM. Then, running 2.4KM will be an ease to you. Same goes for running 5KM, you need to clock at least 10KM.
Run for life!
