its good you went and ran a 5k. Mentally, it will be good for you because you can chart your imporvement. Not to mention that you noticed things you can work on. Obviously, start working those things into your training, especially breathing. Don't worry about pacing so much yet. You can start doing that once you're doing tempo runs in your program (anywhere from 4-8 weeks after you start your program, depending on how you're training is going).
Keep thinking about the decathalon. I think you show some promise in that.
Participant