Just keep running man, endurance is built by doing longer and slower runs or good interval runs. Either way it's a great cardio & a good way to get in shape!
If you're a treadmill type. Try a 20 minute workout saying your speed a 5 (or basically whatever your comfortable pace is), then every min go up .5 on the speed to 7, then go down to 5.5 and repeat so you're up to 7.5, down to 6 and up to a 8, then down to 6.5 and up to 8.5. So that last minute you're just screaming, then do a nice cool down and pop open a bottle of beer
If you're like me and really don't like treadmills one of those cheap ass dinky clip on metronomes work great for intervals and to help you feel your pace until you get used to it.
Good luck on your 5K and don't get discouraged when some old man or woman passes your ass.. I'm not trying to poke fun with that either there are some hard core types in all age brackets so you'll definitely see all types!