Heart Rate Monitor (get it?)
I've been running with a heart rate monitor this last week and I'm here to tell you it is by far the better option when compared to running by pace per km. Why? Because, firstly one can basically just estimate what 1km is when running on anything but a marked track. And, two once you know your max heart rate you can know how to run within the various limits and vary your pace and range more accurately, getting the best out of your training.
When I have been running by "feel" I tend to go too fast on slow days and too slow on fast days. This heart rate monitor is really showing me that my past training has been a bit hit and miss. I think in a month or so my racing condition is going to be so much better because of this new training method.
We shall see.