Running: how can i up my endurance?

i become so wickedly sore and tired after only 15 minutes of regular jogging. i need to be able to jog 5 miles without taking any breaks by the end of the semester for phys ed class. how in the world can i do this?

details about me if needed : 18 years old, 5’5’’, 120 lbs. i’m not incredibly lazy; i get a bit of excersize everyday, like walking to & from school, yoga and stretching in mornings, and playing/horsing around with my nephew in the evening. i’m also starting to run everynight, but i feel no improvement. i just get terrible cramps in my sides and my legs get heavy-feeling after only a few laps.