The first time I ever "went for a jog" I was only six or seven years old. I was inspired to do it by my dad, who ran regularly for exercise at the time. He wore a primitive pedometer when he ran. It was a small, disk-shaped unit that hooked onto the wasitband of his shorts. The oscillation of each stride caused some sort of mechanism inside it to tick, adding one stride length's worth of distance to the total measured distance. As you can imagine, the device was incredibly inaccurate. While you could calibrate it to your estimated stride length, the device had no way to account for changes in stride length resulting from changing speeds or running uphill or downhill. I thought it was pretty cool anyway, and I wore it for my first jog. I ran about half a mile, according to it.
Today's devices for measuring distance covered
as well as speed in real timeare inifnitely more sophisticated, using accelerometers or GPS technology to achieve accuracy levels as high as 99%. I was an early adopter, purchasing the first-generation Timex Speed + Distance device. I've since switched to the Garmin Forerunner, and I use it for almost every run. There are many benefits to using such a device, but their greatest benefit, in my opinion, is one that is seldom talked about: they make you train harder.
In each of my key workouts, I have in mind one or more pace targets that I want to hit. My Forerunner not only allows me to know whether or not I am hitting my targets, but it also encourages me to slightly surpass them, if possible. It's just basic human psychology. You can always dig a little deeper when you're chasing some standard outside yourself than when you're just going by feel. What's more, the device also pushes me to improve my performance from week to week as I repeat certain types of workouts. So, for example, if last week I ran a 10K tempo run in 35:30, when I repeat the workout in two weeks I might pursue a target time of 35:10.
Training runs are not meant to be races, of course, so the purpose of using a speed and distance device to train harder than you might otherwise is not to unleash absolute maximum efforts in everyday workouts. That would take you nowhere in a hurry. The real purpose is to motivate just a slightly greater effort in the two or three workouts you do each week that are supposed to be challenging anyway. The benefits of these extra bits of effort will gradually accumulate over the course of the training process, enabling you to run significantly faster on race day, when time really matters.