Oh, you're quite right, if I practise I will improve. I know I'm nowhere near the actual physical limit of what I'm capable of with training. Right now I'm feeling bouncy and confident so I think I can improve all the way to 5 km in 30 minutes, but after a bad run that seems pretty unattainable. In a lot of ways it doesn't matter, though, any improvement is good, and the prospect of improving helps me to keep it up regularly, which is the main thing.