To run a 5k go out and run right now. Ok how far did you go? Make it easy count blocks, for a very rough estimate just run 20 city blocks and back. Do what you can every day until you reach this goal. All city blocks are different but 20 is about half. Just keep gradually increasing until you can run run 20 blocks and back. If you want to be exact go http://www.usatf.org/routes/map/ and just trace your running route and it will give you exact distance. Put in your zip code for your city map.
I just don't want to quit running and so I keep taking turns away from the way I planned and away from home. Sometimes if I plan a quick 3 mile it might turn into a 10k or even a 10 miler. Its great to explore some new courses this way. Some of my favorite courses have been found this way and anyway it's not good to keep doing the same courses over and over again every day. Sometimes I just hate to quit running.