Improving My 10k Pace

In my quest to complete a sub-50 minute 10k, I've done a couple of 4k training runs at under 5 minute per km and have been pretty whacked by it!! With my new heart monitor, I've been banging on 190 bpm towards the end of it. At one stage I hit 197 bpm and thought my head was going to come off!! It appears to be getting easier and I'm feeling slightly encouraged by that. Let's see how the next run goes (Thurs).