Originally Posted by
Rec
3km in 20 minutes equates to 9km/h. Average walking speed is ~5km/h. Even if you're exhausted, and feel like you can't go farther, at least walk. Eat as many carbs as you can the night before the race, and drink lots of water the day of. Don't eat any solid food for three hours before the run, just water. If the run is long (eg. 1hr), you'll need something like a banana mid way through.
Otherwise, not much you can do really... improving your cardiovascular fitness doesn't happen overnight, obviously. Get in shape, it's worth it.
Edit: just read your post above. For some people, it helps to add rhythm to their breathing. Eg. 2 quick, small breaths, one large one. Another way is to breathe twice through the nose, then twice through the mouth. Repeat.