Over the last few weeks, I have been trying to continue the extensive walking I did during our trip to Israel, around five miles a day, by doing longer fitness walks several times a week. Using a pedometer, such as this Omron:

http://www.rei.com/product/769817/omron-pocket-pedometer

is not essential. An alternative for many might be a smartphone based "app," such as iSmoothRun. For keeping track of distance walked during travel however, I would favor one of the several Omron models, since a GPS-based app uses up phone battery, and really won't work when inside, such as museums and the like. Calibration of a pedometer has its vagaries, one's height alone not the only variable. My ballet-trained wife requires a step length 15% more than what a basic formula suggests, and I need about a 5% adjustment, but even at that, the slower one walks, below a "fitness" pace, the lower the step length should be.

Regardless, a quality pedometer is so small that it fits well with "one bag" travel, and makes an interesting adjunct especially for those who like to keep a journal of their touring day by day. Perhaps other readers might comment on their use...or non-use.

My wife uses the Jawbone Up bracelet, and she loves it. At $129, it's considerably more expensive than a pedometer, but it does a lot, and the iPhone tracking app is pretty good. It seems to be pretty accurate, and doesn't rely on GPS but has its own motion sensor.

I have a slightly different model Omrom pedometer than the one Alan B mentioned. But, I love it. It really helps me keep track of how active I've been. It's also replaced my watch. The only issue for me is when I wear clothes that don't have a pocket. On those days, I don't typically carry the pedometer. (Of course, this isn't an issue when I travel because then I make sure to have a pocket or two.)

<<GPS-based app uses up phone battery, and really won't work when inside>>

NOT quite right. This morning I forgot to turn on the iPhone "Locator," and the app still worked, after a fashion, recorded time, and estimated distance to within 1% of the Omron I was carrying, perhaps having "learned" my average step/stride length. It did calculate pace for segments, average speed, etc. Of course, no map was generated.