To be honest, in running you have to start out with long distances even if it is slow and build up. Running 2.5Km trying to build up to 5 isn't going to work very good. For a test for the Army we have to run 2 miles and the only way to get there is to run at least 2 miles usually 3 miles while you are training.

Thanks for telling me Dmon, what would you recommend that I should do?

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It depends on what you want. If you want to learn to run 5k every other day then start off with 5k. But don't always do the same thing. Monday run 5k as fast as you can, Wednesday do sprints or something really fast paced and then Friday just run for like 20-30 minutes. Switch things up every week so you don't get bored, maybe one day ride a bike for 30 minutes or one day play soccer(futbol) or something that keeps your heart rate up.