Not taking anything away from anyone. But... 5K is a piece of piss. It's easy. I am not a fit person and haven't worked up to it. I run a 5k, once a week. As I could do without losing weight. And I run it at 8mph the whole way. That isn't a hugely fast pace. But it's sub 25 minutes.
Compare that time to Mo Farrah's sub 13 minute time. That is about 15 mph the whole way.
The way to build up, if you need to, is to run a couple of km. Walk for 500m. Then run for another km. Then walk for 500m. Repeat until you hit 5k. Then increase the running times and decrease the walking times. If you can't run 5k after 8 sessions. Then you were not built for running. Everyone should be able to do it, even if you only run at 5 mph.