I'm a recent convert to using iMapMyRun on my phone to calculate the distance of my daily runs. In high school, when my cross country coach would tell me to run 8 miles over the summer to prepare for the fall season, I would make sure I hit that target by going to the track and running 32 long, boring laps around that red, rubber circle. I'm still so traumatized by the experience that I avoid all tracks today. Since then, I've always been a time-only runner, forswearing any distance calculations and paying attention only to my total time on the road. There are advantages and disadvantages to this approach, but still I conscientiously log my daily time in an Excel spreadsheet on my computer.
Since then, however, has come the advent of the GPS watch, which uses GPS tracking to calculate distance and pace, both instantaneously and overall. An expensive piece of machinery, my parents both own one, and I've started borrowing them during marathons and found it extraordinarily helpful in terms of knowing how not to go out too fast. Then,
I started doing long runs with my mother and began calculating them by distance, a more honest approach since I couldn't quite lie to myself about my pace if I began slogging and then would have to remain out on the roads for longer. To my advantage, there has been a much cheaper invention, the mapmyrun apps on smart phones, which can be downloaded for free, which uses GPS tracking in the same manner as a GPS watch. It may not be quite as accurate, but it is a good approximation, and I find it works well.
There are runners who swear by their GPS watches and wouldn't go a day without them. There are definitely positives to this way of running. It keeps the runner honest. There's no lying about running seven miles in an hour when you really only ran six because you were running slower than you thought. You can't fudge the data. It can be motivating to see those numbers and a healthy sense of competition with oneself can be a good thing.
There are also runners who like to run by feel. This is good, too, because checking out and relying on a piece of machinery to tell us what to do and ignoring the signals our bodies are sending us is never a good thing. I would not advise a runner returning from an injury to use a GPS watch because it becomes very tempting to push it too far too fast. In that case, it's better to let your body tell you how far it feels like it can go. Same goes for recovery days after a hard workout or a big race.
And it's always good to check in with yourself and make sure that you're not relying so heavily on data that you aren't aware of how you feel while running since that's the best way to avoid an injury and make fine tuning to your training based on what your body needs.
However, a GPS watch or app can be a great training tool to get motivated or to get real about how far you're running and to track improvements as you're training, which can be eye-opening in itself!